TTL Greece and Bulgaria have a border running for 752 km. In some places that's less than 50km from the sea. Yes the Greeks are free to build up their military and will have British and French cooperation at that but they won't be amused if Bulgaria starts rearming. They will view it as a major threat. Now the question is what's Stabolynskyi's foreign policy going to be. In OTL he tried a genuine detente only to be killed for his troubles. TTL he has survived the coup. If he tries the same Venizelos will be certainly accommodating. What happens with the Serbs now...
Stamboliysky's foreign policy is ostentatively (and sincerely) peaceful ITTL, too. Bulgaria's reinforced defenses concentrate on its Western border, and both Venizelos in Athens and Mihalache in Bucharest are not blind to what goes in Serbia and why Stamboliysky is pursuing the policy vis-a-vis the Serbs. Stamboliysky aims to be allowed to integrate into the EFP; his government does what they think is necessary to ascertain continued UoE support and a path towards EFP accession. On Kerensky's strong recommendation and against loud protests at home, Bulgaria's government has not put up any serious resistance to rather unfair plebiscites which have torn parts off Bulgaria's pre-war territory and awarded it to Romania and Greece respectively. He tries hard to play by EFP rules, which also means his government's protests focus on the insistence that Bulgarian minorities in neighboring countries are awarded their EFP Charter-enshrined cultural rights and are not discriminated against or persecuted, as they presently are in Serbia. Greek and Romanian policies towards their Slavic-speaking minorities are not exactly exemplary, but they're not violent like Serbia's, either. In Bucharest, Foreign Minister Diamandy is quite open to cordial relations with Bulgaria in principle, and while there are still many details to be cleared out of the path, if the current government stays in power in Romania, then 1922 may well bring a Romanian-Bulgarian treaty of sorts and see Romania taking a favourable position with regards to the question of Bulgarian EFP membership. UoE policies in the region are geared towards faciliating such friendly and peaceful relations and unification of the region under the umbrella of the EFP, and currently also containment and isolation of the Serbian threat, at least as long as no secret plan for a "regime change" in Serbia has yet surfaced and succeeded.
As far as Greece is concerned, Stamboliysky's government is the best chance for a detente ITTL, too, in spite of the restoration of some sort of Bulgarian defense. The 1921 clampdown against the VMRO should be good news for Greece, too, for the VMRO is not only the main voice for a Bulgarian "Macedonia" (meaning Southern Serbia) but also for a Bulgarian "Macedonia" (meaning parts of Northern Greece). In this context, the sort of this Bulgarian defense is important, too: It's not a restoration of the old Bulgarian military whose generals might follow old geostrategic reflexes. It's a force nominally under UoE control, and it is currently absolutely out of the question that any UoE force would initiate aggression against Greece. UoE-Greek relations are necessarily always at least ambivalent (Kerensky has his foot very firmly in the Bosphorus and Dardanelles door, in the form of UoE participation in the International Commission for Control over the Straits), but ITTL the UoE has handed over Pontus to a reluctant Greece, and the Black Sea Naval Treaty (see below) has seen UoE naval capacity downsizing and allowed Greek build-up (the easiest way to practically implement this is selling the ships from the UoE's Black Sea Fleet to a new Greek fleet there).
I would fully expect Venizelos to practice a sort of swing diplomacy, trying to maintain good relations with Belgrade (e.g. by blocking too harsh measures against Serbia), London and Bucharest while also not mounting tensions with Sofia and Petrograd. I would expect Venizelos to see the rump Ottomans as the no. 1 threat to Greek security. But I am no expert on the matter, and if you can suggest to me who could be his Foreign Minister, what Greece's position towards Bulgaria would be in more detail etc., I'm very open to suggestions.
Hopefully they are taken down a notch.
I'll come back with more info on Serbia when I wrote about Montenegro and Albania. Currently, except for Greece and to some extent Romania, it is the French government which still hopes to be able to reign in and moderate Racic's new government and who would be not amused at all if it were overthrown with either Italian or UoE help. The EFP, while notionally a big group with many member nations, is de facto always hanging by the thread of a balanced triangle of power between France, Italy, and the UoE, the three big powers in the federation.
BTW. Is there ITL version of Washington Naval Conference?
No success as of yet. Negotiations have been intense in 1919, everyone wanted it, and the UoE wanted it more than anyone else.
In the end, the only thing that has come of it is a Black Sea Naval Treaty, in which the International Control over the Straits is enshrined and UoE, Greek, Romanian and a few Ottoman and Bulgarian naval capacities are defined. As alluded to above, the UoE has been very accomodating, given Volsky's priorities, although the amount of international credit the UoE has received as a consequence has not been quite as high as he had hoped. So far, the victorious powers on the Black Sea are happy with the terms (well, not everyone in the UoE, of course, but so far, nationalist opposition has been scattered and marginalised). The Bulgarian government is accomodating, too, and hopes that by playing nice, they'll be allowed better terms next round. (The treaty is fixed for ten years, 1920-1930, similar to OTL Washington.)
On the worldwide scale, negotiations have hit a dead end. ITTL, British foreign policy, perceiving themselves at the edge of becoming isolated within Europe, and with instability from the Middle East to Southern Asia potentially increasing if the UoE ramped up its indirect support for anti-British groups to an extent that would endanger the stability of the Empire's most important holdings, has placed a much greater emphasis on keeping Japan on their side. The Knox government, on the other hand, has conducted less of a decidedly isolationist foreign policy as Harding's IOTL, and it has seen growing Japanese naval power in the Pacific as a problem. Relations between Britain and the US are slightly cooler than IOTL around this time, and so things don't play out the way they did IOTL. The UoE, as I said, generally pursued an accomodating agenda, but then again, its foreign minister is always prone to bluntness and blunders, so expect Kerensky to screw up such a very complicated matter with some all-too-open remark about British policies in Ireland or Arabia or India or Glasgow or wherever, and voilà, you have no treaty as of yet. The matter is not off the table officially, and the new US president is quite disposed to give a fresh try, but nothing has been concluded.
Any reaction from the local/global IRSDLP to the attempted coup in Bulgaria? I can't shake the feeling that there's somebody who kind of likes that the far-left is now the only party in the Bulgarian parliamentary hemcycle outside of the government ones which is not on Stambolijski's s**tlist and others who are simply fuming at the thought of being the "loyal opposition" to a faux-revolutionary government.
Bulgaria is approaching a situation similar to that in Russia now: Left-agrarians in power with the main opposition being the IRSDLP, bourgeois national-liberal opposition being more marginalised and suspiciously linked with a criminalised terrorist fringe. Bulgaria does not have a tradition of a cruel secret police like the Tsarist Okhrana had been, so maybe the bourgeois parties and the VMRO get spared the full VeCheKist experience which they suffered under Kamkov in 1918 in Russia, also meaning that their marginalisation is probably not quite as thorough and successful as that of the Russian non-leftists.
Bulgaria's IRSDLP was caught by surprise by the coup. In hindsight, the party is inreasingly split along the lines you envisioned. The still dominant current, led by the party's towering figure of authority DImitar Blagoev, is supportive of the "dual strategy" of playing by the legal rules, participating in parliamentary elections (even explicitly supporting the defensive plan with Republican Guards stationed against the Unitarist threat, and calling for IRSDLP members to enlist in them) and engaging in the unions, while at the same time propagandising and promoting militant self-organisation among the workers and declaring that the Bulgarian and generally Yugoslav proletariat must prepare for the moment in which it can rise, shake off the Unitarist yoke in Serbia, overcome the national and capitalist limitations and install a united Yugoslav Socialist Federation. Blagoev sees himself vindicated by the failure of the Rightist putsch, and like IOTL (or: much more so than IOTL!) he is of the opinion that the situation in Bulgaria in 1921 is not yet ripe for a proletarian revolution and that the groundwork must be more thoroughly prepared first.
A current which begins to take more and more concrete shape as somewhat opposed to this legalism rallies around the no. 2 figure in the party, Georgi Dimitrov. Dimitrov is still not openly disloyal to Blagoev, but he is emphasising how Stamboliysky's government is turning increasingly autocratic and that this autocracy will next turn its attention to the Left and that legalism should not be taken to the extreme of naivete, meaning he is advocating less caution in arming, training and preparing Red Guards for the decisive moment, which he thinks might as well come in the near future. Dimitrov also thinks that IRSDLP members in the UoE-commanded Red Guards are a good idea, but he is (tactically uncleverly) explicit about their topmost priority being to prepare the groundwork for turning this force into a revolutionary one...)
Blagoev is an old man. If he dies, Bulgaria's IRSDLP may become an even more "interesting" group.