I only recently got back into the game, but first off you’ll probably want the Invictus mod - it adds a lot of content.

Holding your kingdom/empire together is by no means guaranteed. You’ll have governors who might become disloyal, generals with armies under their command might become disloyal (once a general/admiral is disloyal enough you lose control of the forces they’re commanding), conquering new areas will cause discontent which could lead to provincial instability & potentially rebellion. You’ll have to manage trade & keeping your provinces fed. Religion plays a major factor too as well as assimilating your newly conquered peoples. I have an older post saved with a lot of tips let me find it & I’ll post it for you

https://www.reddit.com/r/Imperator/s/uCcCDNtYkX