I want to do this fully legally.

In Germany you just need to declare to your employer that you have a second job and that it won't interfere with their hours.

I just want to know if its worth it, money/time-wise

then become a contractor, simple.

I'm not sure one can contract one's services to academia like that

Fired for what though? They are saying they’ll do the hours. They are just “working 2 jobs”. Just make sure the contracts allow it and don’t conflict e.g. both require you to turn up at 9am.

What usually happens is people take two L3 jobs instead of a single L5 job, with different benefits and expectations. One of those expectations is not being just a warm body in a seat for 8 hours.

But at lower levels, you could probably automate more stuff too, at the risk of being promoted to a level where you can't.

Both sides would need to sign-off, if they do then OP would be fine.

I suspect that a lot of over-employed people do not get explicit sign-off from both parties. That becomes a gray area.