There seems to be a market for custom 3D printer firmware, consulting and debugging. What should I charge for something like this? I live in Germany and this will be a side quest.

As much as they will pay.

@SortOfTested It seems to be a market like accessories for „audiophiles“. Some have spent three to four times as much on „upgrades“ as the initial value of their printer.

3D printing is a labor of love, but mostly of frustration.

How much are you going to charge them to recommend hairspray as a fastener? How about the plexi case (or cardboard box) to block the dust?

That depends on your customer. If it's a hobbiest charge on the low side.
If it's a company that makes money with 3D printing charge more.

This is how you usually calculate your hourly rates: ("Cost of doing Business" + 15% Contingency if something goes wrong + Your desired profit margin + Your desired salary) divided by the hours you'll work (total per year) = your hourly rate.

Cost of doing business = everything you need to do business. rent, electricity bill, laptop, hardware, subscriptions and licences, marketing and so on. You have to cover those costs or you do not have a viable business.

Your competitive lever are contingency, profit, and salery.