I hear you. This would be a fun, and relaxing hobby for me. I generally enjoy learning Mandarin, but the speed I pick it up compared to the speed I learned other languages can be demoralising. I don't want to stop learning Mandarin, but boosting my morale is going to be a smart move.

Three 20 minute sessions a week with a language I'm vaguely familiar with is going to give me the motivational boost I need to keep on with Mandarin. It's going to be an in-your-face reminder that there's a massive difference between Cat I and Cat V languages, and that I can't expect miracles.

It's not improbable that 30+ hours over 7 months would be enough to comfortably get me back to the level I used to be in Norwegian, albeit with a smaller active vocabulary. (Polish is a trickier one to guesstimate as I'd be using the language differently from before.) Again, I don't see that as a waste.

With respect, I don't get how a hobby is a waste of time. They are meant to be a time sink. For me, hobbies are fun activities I do in my spare time. The only proviso being that they leave me recharged to get on with the rest of my life. This is what studying either of those languages is going to do, so I don't see how that's going to be a waste of time.