games

I'm by no means a Vicky 3 expert, but most times I've found my economy exploding like that was usually due to the same few common issues:

1. My construction sector can't afford the switch to new materials,
2. I changed the production for certain sectors all at the same time (e.g. switching to nitrate for farms, dynamite for mining all at once)
3. A loyal interest group just became disloyal, and I lost their bonus.
4. I was cut off from a trade group/sphere/whatever they're called that I was relying on.

The way I've found to help combat this is by only ever tweaking the individual buildings one at a time at the state level, and never switching all at once. I also make ample use of subsidies to prop up basic industry such as forestry so that I can handle my own basic necessities. Finally, don't be afraid of switching back to more basic modes in your buildings if they can't get what they need. The important mid-game goal is to have a completely self-sufficient steelmaking process and to start churning out both engines and steel-frame buildings. As for diplomacy and warfare... I have no clue. I only ever play Vicky 3 like it's a city builder lol.

Fair warning: I have no real clue how to progress or play optimally. The best I can manage with my limited knowledge of the game is to have my peasants well-fed and happy while still having a positive economy

you can use high taxes, or sporadically stop building yourself, to let porkies build stuff (gotta have porkies tho), interventionism/agrarianism is a must have.

*also sokoto, best korea and mexico are interesting to play (in ascending order of difficulty), sokoto gets invaded like once or twice by france over 100 years, and is relatively well insulated to defend, korea has qing to deal with bullshit (but you have to pay tribute for long ass time), and mexico is kinda well positioned to defend and make alliances against great satan.