I really like this mod but this is the only setback.

Inmis unfortunately isn't updated for 1.18.

The whole point of this mod is to make backpacks that are actually balanced. Backpacks the way Mojang would add them if they were to add them into the game.

Free inventory space is not balanced.

That's intentional.

Is there a way you can make it not do that?

No, because then you may as well use Inmis.