>Every mission from then on had a backup plan in case the shuttle wasn't in a state to return to Earth (this wasn't really the case before then, which is kinda wild). Another shuttle was always ready to launch, with a new configuration of seats to allow for sufficient crew space

Actually the backup plan almost every time was to just stay on the ISS until another Shuttle could be prepared. They only had another Shuttle on standby a couple times, during missions where they weren’t going to the ISS.

>They sent up equipment and materials for repairs in space with every launch, though admittedly the usefulness of that was dubious and the repair kits were never used

Yeah it wasn’t even useful for a situation like Columbia. It didn’t lose a few tiles or something, it had a giant hole punched into its wing.

There’s no fixing that in space. So I personally think they focused on situations they could theoretically fix, even though those situations weren’t what happened to Columbia.

Worth mentioning, this is all particularly fresh in my mind because of a recently released video by the excellent Classic Aerospace History channel on YT, "A Brief History of the Space Shuttle". It's two hours long and provides a reasonably detailed overview of the program, would recommend if you're into that sort of thing: https://www.youtube.com/watch?v=WtmOVxcga-Y