If 1. This is not fun, 2. There's no guarantee it will pay off, honestly you just shouldn't do it.

Some things need to get done, at least short term, sure. But if that's structural, some major chances should be done.

If a side project is neither a source of learning, nor fun, nor of opportunity (and thus "it doesn't pay off", in your words) - then just don't do it and find something else.

I'm a fierce believer in intrinsic motivation. Extrinsic motivations don't hold up. You're set to give up sooner or later.