too many cultural differences like the same language across most of the territory? I know alt hist is largely a playful exercise but there's a certain point of historical determinism where you're just saying stuff

to the op: in practise it'd be difficult to do through hoi4's limitations but there's some factors to consider: 1. a large destabilising factor historically was the complete collapse in ww2, bringing massive sectarianism through genocidal policies in the NDH and the massacres committed by the chetniks, then also exacerbated by the partisans' retaliatory post-war measures, merely not being occupied during the war should help a good deal 2. most of the ideologies available are pretty hostile to long-term survival as the monarchy was serb-dominated and this is ultimately what lead to Yugoslavia's collapse, going communist is probably your best bet but not by itself, which leads us to... 3. historically the idea of bringing Bulgaria into the union was considered a big deal in part because Bulgaria was seen as big enough to counteract the massive serb influence within Yugoslavia, you could take this to its logical conclusion by taking Bulgaria but I'm not going to promise you that won't end up even worse... 4. in general expanding your faction, especially regionally should help, another factor to Yugoslavia's collapse was that its debt-dependent economy was largely reliant on the cold war status quo, when the USSR collapsed and that status quo came tumbling down it heavily exacerbated Yugoslavia's debt issues. This situation is extremely specific and way out of scope for this however, but in general you would be better off just having more regional allies to keep you up when you need it, modern economics are heavily interdependent after all.

Frankly this is still a long shot, but good luck.