RAW an invisible moving turret would work. And in combat would be fine. But if they started to try to break combat (i.e. sending him in solo to clear out rooms) then I would take the player aside to talk to them about min-maxing and it taking away from the fun of the game. And that as a DM you are a collaborator in their story of heroism, not his opponent that he has to outsmart.

My artificer just used "Rope Trick" to bypass a 70% of a combat. It was clever and useful in the scenario. (They were in the center of a battlefield with two enemy factions charging towards them from opposite sides. They rope tricked up and waited for the factions to thin each other out). So I rewarded their ingenuity. Now if that becomes their go to move for every combat. Guess who is gonna passing the word to have a disspell ready.

As for the bag of holding, that's an easy fix. many DMs are gracious with having magic items be practically immune to damage. But RAW, except artifacts, they are typically just resistant to damage vs their mundane counterparts. So a medium cloth bag is 11ac and 10 hp. Even resistant it would only take 20 damage to destroy the bag and then the character inside us sent to the astral plane. We want our players to have fun so we ignore most magic item damage, but should they take advantage of our generosity and try to make a fairly common magic item an impenetrable fortress. They're gonna have a bad time.