Card battlers without continuous payments

Looking for recommendations of "traditional" battle card games where you don't have to buy new expansions every few months. (Being able to grind the cards with playtime doesn't count.)

I know that there are some decent open-source MTG rule-engines which makes MTG a viable option. (xmage, forge)

Then Slay the Spire comes to mind, possibly there are other good solitares as well?