I think that doing the plain background pieces will make your art more accessible. You may not enjoy the finished product as much, but your art will be available to more

Possibly in the short term, but long term, working on art that they don't enjoy as much is likely to leave an artist feeling burned out and demotivated, which is no good for them or for people wanting to commission them.

I'd say if you're thinking about it, sure, open a couple-few slots, give 'em a try and see how you feel. Maybe experiment if there's some way you could do a liiiittle more with them that would make working on them more satisfying, and bump the price up accordingly. It seems like $150 - $250 is a price threshold for a lot of people, and you're already north of the lower end of that, so why not see if there's a version of cel-shade sketch in the $200-$225 area that you'd be more excited about?

I’d say doing the cel-shaded pinups as a once in a while thing would be fine if you felt up to it, but ultimately, do what you enjoy doing most as an artist!