Bonus points if you can outlevel the current encounters by grinding enough, and for turn-based games.
Warframe. Don’t spend any Platinum. Played that game for 2 years straight before I finally attained every gun, every frame, every pet, every skin.
CrossCode. Grinding is never necessary to progress, you can blitz through the main story if you want, but there are tons of sidequests to sink your teeth into. If you want to just let loose and farm, there’s a mechanic where killing enough consecutive enemies quickly triggers a state enemies respawn faster while dropping more loot (and cool music plays).
XCOM2 with Long War of the Chosen Mod.
Turn based, Grindy, Unforgiving.
terraria master mode is probably my favourite grindy game
Slightly tempered recommendation for Albion Online. It certainly scratches a grinding itch.
Any Disgaea. Max level of 9999, later games go further. I know in several you can go inside your weapons to level them up.
Pokémon. Any of them really. I’m partial to HeartGold/SoulSilver myself.