The collapses you need to handle are Russia, France, Turkey, Austria, Germany. It’s a tough problem.
All of them led to powers surrendering, except France which only lost the ability to attack for a while. Russia is arguably two collapses, with the first being a bit like France’s collapse, and the second a proper collapse. Probably there are two collapse thresholds for each power, at the first they lose the ability to attack, at the second they go bottom-up.
If you make e.g. the Russian Revolution happen on a timetable then you discourage the Germans from attacking Russia. Don’t want that.
But if you base collapse on casualties then you encourage the Germans to go all out against one of their enemies. That isn’t quite what happened historically - they attacked some in the west, even after 1914 - but it’s closer to reality.
Historically loss of territory (e.g. Syria and Palestine for the Turks) seems to have mattered. And the ignominy of losing a battle to Italy seems to have depressed the Austrians. :-)
US entry time could be affected by use of U-boats.
You need rules for Italian entry too? Plus Bulgaria, Romania, etc…
Germany may not have collapsed. Maybe the player just said, “OK, you win, let’s try the WW3-in-1946 scenario next.”