So far I don't exactly know what features are missing compared to the current windows version. Is there maybe a comparison chart or something like this available?

The existing linux version looks very promising and as you say it is stable and usable.

But I believe it could scare potential users away that it 'seems' the software is outdated and not maintained anymore. Maybe that could also be another reason that additional linux developers are not more interested in contributing to the project.

Can a developer maybe outline what skills are needed to contribute to the project from a technical point of view (programming language etc.) and what could be a starting point for a developer (is there a to do list etc?).

Unfortunately, I suppose I don't have sufficient programming skills for a task like this right now. But I am willing to learn and contribute back in the future if possible. Also, maybe it is easier to find skilled linux developers if it is more transparent what specific skills are needed and what exactly has to be done.