This is what I have now. Level 1 to 4 are confirmed in setup. Since I am setting up everything except for Serbia the coming days, that gives me time to do some more (soul) searching.
Level 1: National Division
Level 2: National Division
Level 3: 4 Interrepublic Divisions
Level 4: Republic Division (1 or more) 1Slovenian Division, 3 Bosnian Divisions, 4 Croatian Divisions, 1 Vojvodinian Division, 1 Montenegrin Division, 1 Macedonian Division, 2 Serbian Division (Serbia and Kosovo)
Level 5 confirmed: Regional Divisions (1 or more) 2 Slovenian Divisions, 7 Croatian Divisions, 1 Vojvodinian Division
Level 5 unconfirmed but likely, based on available information: Regional Divisions (1 or more) 3 Bosnian Divisions, 1 Montenegrin Division, 1 Macedonian Division
Level 5 unconfirmed and educated guess: 3 Serbian divisions, 1 for Belgrade, 1 for Kosovo, 1 for rest of Serbia. The Kosovo one is pretty certain based on level 4. 2 The Serbian ones are educated guesses, but a version with Belgrade, Kosovo and a Serbian east-west split is one that could be possible for 4 divisions as well