The big problem with any CP win is the winner will want to loser to carry the burden of debt for the war.
True. But there is one big difference: OTL Germany lost. The armistice already required them to give up any favorable military position, so further resistance was impossible. With a POD after 1917, the CP could never achieve such a victory. All they could hope for is status quo. If we assume that they knock Italy out and get a peace in the east, this might be enough for the western powers not to sue for peace - they wouldn't. After all, Britain would always be secure.
But they might be willing to negotiate. Now if negotiations start among equals, noone would have to pay everything. Only the looser pays, hence Britain will never pay. The only chance for the CP to get paid is a full defeat of France AND the British expeditionary forces, which is not possible. So IMO, the question is whether the CP could get some victories to get the western powers to the negotiation table.
That's what the french did with the ToV and it is most likely what the CP will do if they can force a peace.
True: if they can FORCE a peace, which is impossible.
As for the colonies they are gone until the next war. GB and japan won't give them back and any ceded to the CP by France and Italy will be conquered by the British and Japanese.
If France is defeated and Britain carries on, true. But I can't see this to happen. I think the only possibility is a negotiation, started by equally exhausted enemies, initiated by recent CP victories, due to which the war seems to last even longer for the western powers.
So my proposal for the best case for Germany:
At the begin of 1918 Germany gets peace treaties with Russia, Ukraine and Romania. Western powers totally overestimate the influx of supplies for the CP.
Then, after a successful counterstrike in Italy, the siege of Venice begins, the CP approach the river Po, as the Italian army, exhausted and frustrated by this surprisingly successfull attack, retreats. In Turin and Milan and other industrialized areas, communist uprisings start. The government responds sharply, quickly executing rioters. Then the Germans cross the Po, new troops start a mutiny and build up bolshevik-like councils.
(I don't know if a red uprising in Italy is realistic. Yet it doesn't need to be a full scale revolution, just enough to first force an armistice between Italy and the CP and second to scare the western powers of the Red Menace).
An assassination attempt by some mad communist/anarchist to some minor official in France increases fears of the bolshevik threat.
In May, the remnants of the Hochseeflotte get a minor victory in the North Sea, without any militaric value, but it is a great success for propaganda, war weariness in Britain suddenly roses.
Karl of Austria announces his wish for peace, the Americans are hesitating to come to europe, as it seems the Germans are gaining power and the war would last for years to come.
Germans and Austrians agree on terms for an armistice, the western powers, anxious of the reds and under pressure from America, agree.
The Germans retreat into Belgium, into a prepared fortified frontier, as a first condition, and Britain accepts some food supplies being delivered to Germany.
The congress of Amsterdam starts, when it ends in 1920, German, British and French troops are already fighting alongside each other in Russia against the Communist forces. Lorraine comes to France, Luxemburg to Germany. In Alsace a poll shows favour of staying German, yet some bordering regions are passed to France. Germany pays a little for Belgium and France, and grants trade privileges. The peaces in the East are revised, but be kept in favour of Germany. Germany looses its asian and pacific colonies and parts of Kamerun. The Turkish empire is partitioned between Germany, France and Britain.
Only a few years later, AH dissolves, Germany taking rfirst Germany and the Sudetenlands, then Bohemia and Moravia as well as Slovenia. France protests, Britain not that much.