Thank you very much. What you write confirms what I expect. I'll see if I can find something equivalent to Pragmatic Marketing in Germany to get that leg up.
Thanks. Some focus is a common theme in the comments here. I'll try to follow that advice, though I'm not sure I want to go much deeper in programming.
"join a consulting firm" That is good advice, sounds like a good fit. I don't feel expert enough to consult freelancing, but with the backing of a firm it should be more easy to figure out where my weaknesses are.
I really have to get over the reverse Dunning–Kruger Effect (I know enough to know how much I don't know). In practice this is not an issue: if a problem is solved is farely easy to verify, but making statements is something else.
I am valued, but the startup is really low on money (because we try to get our project done, not do much client work). As some compromise I now do it just 3 days a week and partly remote.