I am once again asking for disposable "traditional" upper stages that don't need heat shields, flaps, or header tanks.

Reuse of the upper stage is probably a dead end. It doesn't really make sense to bring all that weight to orbit and then try to bring it back. Maybe it would make sense to salvage the engines and return them in bulk for refurbishing, but even that is doubtful.

That said, the most likely solution to the heatshield issue is a liquid, gel like heatshield that gets shed on reentry and can be easily refilled.

ngl in the long term it might cooked be if someone can make nuclear powered rockets. by long term I mean in the next 10-20 years.