ismine function returns false...
So nothing, I was soooo sure, but wrong -_-
Thanks everybody for the help!
I assume the function look up your address only in the listed ones and therefore you will get false as result. You could try to create a raw transaction and let electrum only sign it for you.
Just an idea, don't know if it will work.
Resizing the "gap_limit" if probably a better way to go tbh. Changing it to 100/200 and seeing if that same code returns false would probably do better.