I would also appreciate an option to save builds - even the option to save only one would improve the QoL. I could save my current build, try something new, and if i don't like it I can simply revert back.

it would allmost certainly encourage more..casual ..players to play around more , if they could easier revert to a known build , and would over all encourage people to experiment even more i think

This quality of life change has literally nothing to do with "...casual ...players".
The point is to keep change fee (35k gold and 2k affinity) but allow people to save 2-3 builds that they want, because trying new builds becomes a pain in the ass.

Changing whole your gear and skills completely inside an expedition isn't too casual for you? I would get it if that would be allowed only in simple mode. But in expeditions it's quite OP and imo would be way better to lock both gear and skill inside of it (or AT LEAST inside of an Untanted run). It is an end-game activity, after all.