Would have been more useful in Avengers: Endgame, since the army and Thanos without the gauntlet can't really combat it.

I believe the reality stone allows Thanos to escape another dimension (or simply counter the original spell as mentioned) and remain in the real world.

It's a mirror of reality. The reality stone could break out of it, and the Space stone could perhaps create a portal out.

But maybe the the stones could destroy the mirror realm

I think he doesn't do that because thanos have the reality stone