In the midgame after 1830 I run every time out of money because the minor nation vanished and nobody is trading with me anymore. Any advice how to make decent money lategame?

Once the minor nation is at max relationship, keep pushing to make it your colony.
Otherwise conquer some minor nation capitals, then they will still be in the buying market sometimes.

Get a few colonies of your own.
Start with a trading consulate which will improve your relations for every trade you make with that nation.
Buy their resources. 1 of each resource they offer will be enough. Buying more than one unit will not increase relations faster.
Use subsidies to become their favored trading partner. (Hold control when assigning a subsidy to have the game set the minimum required to become favored trading partner.)
Build an embassy when you can afford it
Offer a pact for a big relations boost.
Once relations are in the dark green, offer a 10K grant and annex them.

Buying their land makes sure they allways offer those resources but iron, coal and oil need to be developed once you buy those land tiles. (as long as those tiles are not owned by a major power they produce 1 resource each)