QoL Change - TB

There's a good reason behind why this isn't a thing and you indeed touch upon it- the main one being the game would need to remember what team and relics / mods you initially completed the battle at and remove them from use or factor in that the mods could have been moved as doing a battle gives deploy points and relic level and mods affect this.

This needs to be done so that you can't cheese it by  taking the sim and using that team somewhere else (Eg Initial win is GLAT and GLAT can be used in 2 zones).

Doing it fairly (again you do mention this) would be very complex and also need a huge amount of data to be stored and kept updated.

Would I too like this? Yes, but its a huge undertaking.

TLDR: Its nowhere near as simplistic as "done battle, now it can be simmed".