Yeah same in Norway.

For example if someone asks me if I want to go somewhere with them at a specific time, and I would like to go with them but I already plan to do something else at that time, then I would say “nja” and while I say that think out an alternative suggestion for when we might go.

Another example is if I talk with someone and they say something that I agree with but then say something I disagree with. Then I would say “nja”. In this case it would be similar to English where you might say “yes and no”. And then state what I agree with and what I don’t agree with and why.

I would say /care, I don't care, either way, both fine with me, or "its nuanced" or "its complicated". The latter two invite curiosity and discussion, whereas the former one and its, ehh.., relatives (for lack of a better word) are more firm in the carelessness aspect therefore being more passive from my part. You can also say no and nod yes, as Torvalds famously did in response to a question concerning NSA.

I sometimes use it at work when asked if something can be done and the answer is yes but it's unlikely to be worth the effort

Nice, we have Jein in German, which is a Portmanteau of Ja and Nein (yes and no).