Your best tips/forgotten mechanics

So I've got just under 1000 hours in EU 4, love the game and always thought i was a decent player and knew the game well.

Until like 3 or 4 weeks ago when I saw a post on this sub about upgrading your ships, I had never used this feature and started to understand why my navies would get smashed later on in the game!

Im now thinking what else am I missing, what other simple features or mechanics have I never used and thus holding me back in my games!

Please share away with things you've only just discovered, or have been using wrong in your games?

I'm almost a 1000 hours in, and now think I know nothing about this game!

Help!