Looking for a grindy game

Any Disgaea. Max level of 9999, later games go further. I know in several you can go inside your weapons to level them up.

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.

Warframe lets you outlevel everything if you grind enough, but is a twitchy third person shooter.
Tactics: Ogre Reborn is a squad turn based RPG a la Final Fantasy Tactics, and while it has level caps to stop you outlevelling the mission the first time you do each one, you'll be going back to them several times each for the endgame and can very much outlevel them then.

XCOM2 with Long War of the Chosen Mod.

Turn based, Grindy, Unforgiving.

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).

terraria master mode is probably my favourite grindy game

Slightly tempered recommendation for Albion Online. It certainly scratches a grinding itch.