Triple paneling of individual interior walls

You can safely hang 2 elephants on 15mm OSB (provided you get the appropriate screws) and have an import permit for elephants.

For soundproofing, you need mass and/or preferably a floating frame that is decoupled from the actual wall (you were mainly concerned with structure-borne sound - right?) However, that requires space and a sufficiently large and well-filled wallet.

you can hang 2 elephants on 15mm OSB without any worries (provided you get the right screws) and you have an import permit for elephants.

I am a unique piece, but don’t have to be imported to Germany anymore ;-)

Ok, then I see no reason why I should switch from 15mm OSB to 22mm OSB or is there a reason that I have overlooked? It certainly won't cost that much more?

One more question about the 2nd drywall on the 1st drywall wall:
Do I even have to embed a mesh tape at the transitions? According to my understanding, the cracks rather come from OSB to the 1st drywall since different materials and therefore different expansion.

One more question about the 2nd drywall on the 1st drywall wall: Do I even have to embed a fiberglass tape at the joints? The cracks, as I understand it, rather come from OSB to the 1st drywall, due to different materials and therefore different expansion.

That is probably true as well, that the movement mainly comes from the material transition. But also consider that the second layer will become your visible surface, since it apparently will be directly primed and painted. To me, that feels too "risky" that a crack won't appear. To me, that one roll of fiberglass tape would definitely be worth it.

If it is supposed to be neat, I would definitely embed plaster mesh tape, and then fully plaster and sand it. Personally, with the concerns in mind, I would rather lean towards a construction-side solution during the wall assembly and not a "makeshift solution" afterwards with at least an unknown effect, tending instead towards "better to do it right from the start," but everyone is different.