>neither Super Heavy or any future Starship tanker will be rapidly reusable that much is clear by now

Funny, because I would say for the first time there's a clear path to rapid reuse.

What insurmountable problem(s) do you expect?

Pretty much everyone is heading in the direction of orbital fuel depots and orbital refueling.We're getting pretty good at launching rockets and docking things in orbit. Prior to Starship, ULA was considering having Vulcan's upper stage have refueling capabilities, but their parent company, Boeing, saw it as a threat to their SLS money and tried to get the associated engineers fired. All HLS proposals involved some sort of refueling and reuse capability, and both accepted proposals involve orbital fuel depots.

The only reason NASA hadn't already been investigating the relevant technologies is that politicians threatened to outright cancel space science funding if NASA so much as mentioned depots (the word was actually censored in documents from NASA about HLS until the relevant senator finally retired).