I am in the position to acquire some land with a forest on it in Germany. What could I do with it so it pays for itself in the long run and pays a decent and sustainable profit
Are you looking for this to be passive income? Or a full time job?
Clear cutting a half or full hectare and doing intensive market gardening can almost always turn a profit. But it's a hard industry requiring lots of knowledge and tons of work/time (think 6 days a week for at least half the year).
You can utilize the rest of the forest as sustainable forestry, using the cut wood for wood chips for the farm, and interplanting critical native wildflowers to boost pollinators.
Plenty of space to do an apiary (bee keeping) for extra income selling the honey.
And on the side you can do mushrooms like the other commentor said. It can be a relatively low amount of work once you've mastered the technique.
And all of this can be a net benefit to the land. Losing a few trees can open up a forest to allow better long term growth, increase top soil over time (via organic no-till gardening) and support native pollinators via human-maintained wild spaces.