"Using all starter cards will earn 0 rewards
Using a few starter cards will still earn rewards, but a reduced amount"

This is a good change but risky. One of Splinterland's appeal is it's one of the lowest entry NFTs so you can start it on a budget. If you're going to implement these two changes and yet the RENT > Rewards, it would probably turn away new players. Because, logic, why would I rent if I would lose money due to low rewards. If I rent a total of 10 dec/ day and earn 2 dec/day, then what's the point of renting?

Either you should increase the rewards to the point that you can actually earn from that OR remove the entry point of $10 for playing the game. I think the idea of implementing Spellbook is for players to be rented the cards in return. So, with spellbook and those changes implemented, Splinterlands is technically creating a membership or registration fee without any rewards for it like, "Hey, if you want to play splinterlands, you need to register the game for $10. Oh, you want to earn as well? Sorry. That's not included in the $10 you just paid. That $10 is just to see what's inside the game, not including the earning part."