Why not a more Darkest Dungeon approach.

In Darkest Dungeon you have a clear difference between Town (Camp, Open World) and Dungeon Zone.
Enter a Dungeon means no fast travel, restock or exchange Party members.

So a Dungeon can be a 2 long Rest Dungeon and you can use your Spells 3 times.
And every time you use a Camping Ressource or reach a certain point it Saves the current monster population.
After 2 long Rest you have no "Save Points" anymore and must clear the rest with onyl 1 usage of Spells.
Leave the Dungeon with no Save Points and long rest in the camp means the Monster Respawn every time.

This had the side effect like Risk versus Reward. Exhaustet Party Fight vs Monsters means a less Monster after "Save Point".