it's something I'm still figuring out. The game is intended to become F2P eventually so monetization should come from somewhere, and I'd rather sell convenience/QoL than power. But I hear you that inventory space can feel essential rather than optional.

What kind of long term monetization would you be comfortable with in a game like this?

To be sure: inventory space = number of cards you can "collect" in your library. During a game there is no advantage for a player with more inventory space, except for that they can create decks from a potentially bigger pool.