Steam Deck

That is a lot of contradictory sets of requirements. If it is important to have on the deck then it is going to be trivially searchable online. Something that is niche that others are not really doing is going to be very subjectively interesting or useful. That makes it impossible to recommend anything without violating one of those requirements.

Instead here is some advice for finding project ideas: Look at your own interests/hobbies/things you need to do and start taking note of problems you encounter, grievances or annoyances you have or just things you think could be made/done easier. Out of those you can look at ones that you think a steam deck could help solve and from that you can start to investigate ways to use the steam deck to solve those problems. That is essentially how you find niche and interesting/useful things that are specific to you to work on. It can take time, but the more you think about it and write things down the easier it becomes to find projects to do.

Things that I can just easily stop.

Technically any non-online game will work since you can just put the steam deck to sleep with the tap of the power button when ever you want and resume later on. It takes a couple of seconds to go to sleep and so the only times it is annoying is when you are directly in the middle of some action - which is generally easy to avoid in most games if you know you are coming up to your stop.

Personally I have been playing monster hunter world like this which works quite well - especially since there is quite a bit of less action packed stuff you can do between the main story line.