Factorio ought to keep you busy for about 500 hours even without mods

You could probably play the SCS Truck Simulator games basically indefinitely.

Vampire Survivors is an absurdly addicting Deck game, and can be considered grindy if you go for the eggs late-game.

Deep Rock Galactic is a good one too, if you're okay with no aim assist and playing solo or with randoms.

Gunfire Reborn, Hades, Vampire Survivor, Cyberpunk 2077, Carrion, Slime Rancher 1 & 2, Stardew Valley, Warframe, Death Stranding, Wreckfest, Orcs Must Die 3, Castle Crashers, ARK: Survival Evolved, and Dead Cells are all over the map genre wise but all have grindy elements that may or may not vibe with you.

No mans Sky is pretty good. My buddy and I teamed up and make a series of production facilities across a network of planets in order to provide enough materials to manufacture the most expensive items in the game.

There was no reason to do that considering how charitable people are, but we just did it anyway.

You can also grind for the best ships and gear and stuff, or build a base on a planet or on a space freighter.

Factorio is indeed a great choice. Whenever you're ready for more Monster Hunter, Risse+Sunbreak is also amazing (playing it now myself). ARPGs like Grim dawn and Chronicon also give plenty of playtime if they're your style.

Path of Exile has a lot of good grind to it. Every league added mechanics, and a bunch of them went core over the years. Also, if you haven't hit the Guiding Lands in MHW, the grind gets real there.

My brother has 500 hours in some Disgaea or other.

Minecraft: Dungeons, easily. I usually play that with a show on or something, it's mindless fun

How do you define grind? I went from zero to sixty hours in Rimworld in the span of a few weeks and IMO it plays quite well on the deck (not perfect, but surprisingly well for a traditionally M&K PC game).

If you're fine with any genre, I've found Survivorlikes to be a good bit of time killing fun on the Deck. Vampire Survivor is the obvious choice, but Halls of Torment is a much more polished interpretation with the look and feel of classic dungeon crawlers like Diablo.

Warframe. I played it for 1400 hours, and I can attest that it is grindy.

But it's still fun, and there's a lot of content for a F2P game. You can even earn the premium currency by grinding (and a few events throughout the year). It's Deck Verified, too!

ESO can be grindy depending on what you do.

Hollow Knight is pretty good.

GregTech: New Horizons (A minecraft modpack) should keep you busy for… about 8000 hours?

Monster Hunter is my favourite grindy series, and World just recently got verified

Siralim Ultimate

Crystal Project. Classic final fantasy but different.

Cyberpunk 2077, 250 quests. I haven't started phantom thieve yet.

Warframe

Have you tried the Ys series of action RPGs? Ys VIII is a lot of grindy monster whacking fun and runs great on Deck.

If you like games with XP, Baldurs Gate 3 runs great on the deck.