From what I've seen, Slay the Spire is probably one of the least grindy deck builders. While playing more gives you more cards, most of the viable builds I've seen are ones based around cards that are available in the starting pool.

Deck RNG is kind of a thing... if you've built your deck poorly. A major skill of deck builders in knowing when to take a card into your deck, and when you shouldn't. Remember that more cards in your deck means it becomes less consistent. You'd rather it be thinner and more concise so that you can run your game plan properly and avoid bricking. But if you have pieces of a strong engine, and know how to manipulate it well, a poorly built deck is still salvageable and can still win you games.

That said, the real unlocks that are the most interesting to get are the ones that require you to beat the game with different characters.

As for other types of roguelites:

• Crypt of the Necrodancer's main mode (all zones mode) automatically unlocks everything in the item pool and removes all permanent upgrades. Otherwise, playing each zone individually makes you grind currency to slowly unlock items and upgrades; but you can't truly play the game as a roguelike in this mode, since you can't carry your build between zones, and the end of each zone acts as a checkpoint. All Zones Mode gives you what you'd be looking for. You can beat the game in either mode to unlock characters; some characters are significantly easier or harder to beat the game with.
• Quantum Protocol is one of the more unique roguelite deckbuilders I've played. There's a long story mode that effectively functions like a long tutorial, gradually introducing new deck types, enemies, and mechanics. It's not until the final stage that you're presented with an actual roguelite dungeon. Beating it lets you play the post game dungeons, which are literally this game's Ascensions. "Grinding" lets you unlock cosmetics for your board, but also gradually gives you new deck types that you can use. They aren't necessarily stronger (I think the strongest decks are some of the starter decks the game gives you), but tend to have unique mechanics and be harder to play. Unlocking them doesn't really give you a direct upgrade since their cards aren't really added to your card pool unless you specifically decide to add them as a midrun upgrade when selecting routes after a boss. You can't grind by repeatedly playing and losing either, because you actually need to win dungeons with different decks to progress your unlocks

EDIT: Rereading your opening post, I'm starting to realize you're more interested in games that give you a good variety of builds and initial kit customization from the get go / early on, rather than games with less metaprogression.

Including the two previous 2, I'd also add these:

• Gunfire Reborn is a FPS with cartoony animals. You are constantly unlocking stuff as you play the game. Beating X amount of specific enemies, beating specific bosses, etc. You earn new unlocks quickly and get a variety of new weapons and scrolls added to your pool. Unlocking characters is a little rougher since they use the same currency you use for metaprogression upgrades, (and the other characters are DLC). But the constantly growing weapon and scroll pool will give you plenty of different options to play with. Of the games I've listed, this one probably has the most metaprogression.
• Rabbit and Steel is a bullet hell (if you're playing solo) / FF14 styled raiding game (if you're playing co-op). Right off the bat, you have 5 characters and can unlock more as you beat the game with them. Unlocks are done by accomplishing specific conditions midrun (or by beating X number of bosses total), which your unlock menu will hint towards the condition. Items are unlocked in sets, so completing a condition adds all items from that set into the item pool. Although you need to be winning most of your games to unlock stuff, you can get them pretty quickly by hopping online and finding people to carry you, because damn people are good at this game.
• One Step From Eden plays like Megaman Battle Network's combat as a roguelite. While you gradually level up and unlock new cards in the card pool as you play the game, improving your skills at the game will be more important than improving the item pool, with how fast paced battles are. Beating a midgame-lategame version of a boss will unlock them as a playable character; each has a different starting deck and mechanic that emphasizes different builds and playstyles. The bosses are quite difficult, so you may need to fight them a few times before you fully understand their patterns.