So now that I can actually analyze the situation a little, I would propose two important focuses during the civil war of 1321 - 28, the first concerns the figure of Syrgiannes Palaeologus ( who negotiated for both sides, but then ended up assisting Stefan Dusan to conquer part of Macedonia, for which he was then assassinated by Constantinople, once Andronikos III definitively obtained power and was able to concentrate on the European front ) the second instead concerns the survival of Andronikos' first wife ( Adelaide of Brunswick, who died in 1324 ) who was on excellent terms with John Kantakouzenos, compared to Anna of Savoy, who instead looked at him with enormous suspicion, so as to avoid / mitigate the tensions between the court factions which then led to the open clash ( in which the Turkish beilicates also took part for the first time ( who devastated Thrace and southern Greece ) and causing them to quickly gain prominence in power politics in Byzantium ) now as regards how Byzantium could possibly evolve in the years between 1340 and 1350, I can see 3 key points of the foreign policy of Constantinople : the first is to stem the strength of Serbia in this period ( which under Stefan III was the real regional power ( if we limit ourselves to not considering the Hungary of Louis I ), but more due to the sovereign's capabilities than anything else ) perhaps by creating a network of alliances with Bulgaria and Hungary, the second instead concerns Asia Minor, where to maintain the few remaining possessions ( but fundamental from an economic point of view ) I would see Constantinople ending up supporting the Karamanids against the Ottomans and at the same time keeping calm down the claims the Mega Komnenos of Trebizond, propose a matrimonial alliance with them, to conclude a careful diplomatic policy with the Latin powers will be important, both to avoid any ambition to undertake a conquest of the Rhomanoi territories ( mainly Epirus, which was coveted both by Naples but also by Venice and the remaining Frankish - Greek princes ) now probably ATL John V, growing up and gaining experience of government under his father's tutelage, will not become the sovereign that Otl counterpart was ( at best scenario ) but it must be remembered that the Black Death will be an enormous blow to one's abilities ( limited ) of the state, but if the fortress of Gallipoli is not structurally weakened and then handed over to the Turks like Otl ( which technically was easily retaken on behalf of Byzantium by a small crusader force led by the green count of Savoy, only to be "given away" again into the hands of the Ottomans by a pretender to the imperial throne a few years later, therefore with a vaguely more stable Empire , it shouldn't be that difficult to keep it under state control ) , there is a good chance of seeing Byzantium find itself in a slightly better position than Otl ( barring any civil wars, which are not to be ruled out entirely ) especially when Serbia began to fracture internally after Dusan's death in 1355, furthermore I would look for a more important bride and above all outside of the internal power struggles of the state for the future John V ( which was the result of the attempt to pacify the situation after the two rounds of civil war in Otl ), possibly who is considered superpartes by all the participants in the Western schism, to conclude I would still try to play the maritime republics against each other ( also because in OTL Genoa became a serious problem in this period ) but personally I would aim to slightly favor Pisa, rather than the one with less interests or territorial ambitions in the region respect a Venice or Genoa
P.s
I hope I was able to give a satisfactory answer to your questions