Although Yugoslavia's army should roughly equal yours (although most of the time, Yugoslavia focuses on its airforce and only builds 3-5 infantry divisions, making your job easier. Only declare war on Yugoslavia after no nation is still guaranteeing its independence. You should have in excess of 30 divisions by 1941, as well as an HQ. Focus on industrial technologies and infantry/land doctrines as long as you only have two slots - the rest can come later. Try to produce extra supplies so you can trade them for other resources down the road. Try not to build any 1918 infantry at the start and focus on upgrading and repairing your army until basic infantry has been researched, at which point you should starting building fairly long queues of 1936 infantry.

Since you can't do anything for the first 5 years, that means you have to build up you army steadily until the time comes to go to war. Alternatively, you can stock up on supplies and trade them for whatever else you require. You also have to deal with your shortage of resources - Germany is a good trading partner, as it needs the metal which you can provide. Although you are surrounded by nations which are just aching for a whooping, they have their independence guaranteed by the United Kingdom or Germany.

You probably won't be able to go to war anytime soon with Hungary.

No navy (unimportant, if you don´t have a port).

Low starting IC compared to some neighbors.

All neighboring nations have their independence guaranteed by at least 3 countries each.

Right in the middle of Eastern Europe (to make conquest easier).

Hungary has a lot of going things going for it, but there are some negative aspects which impede its success worldwide.

Hearts of iron 4 hungary