Really cool concept — combining kart driving with Vampire Survivors-style auto-shooting is something I haven't seen before. The premise of gnomes reclaiming a monster-overrun world from underground gives it a fun, scrappy underdog vibe.
A few thoughts from reading through the page:
The upgrade variety sounds great (guns, rockets, bulldozer builds). One thing that could make or break the roguelite loop is whether each run feels genuinely different. Do the upgrade paths branch enough that you'd build totally different karts between runs? If every run converges to "stack as many guns as possible," that might get stale — but if there are viable melee/ramming builds vs. ranged builds vs. mobility/flight builds, that's a really strong hook.
The terrain hazards (cliffs, bridges) with monsters chasing you is a really nice tension mechanic. Driving games already have spatial challenge, so adding enemy pressure on top should create some chaotic moments. Does the terrain change between runs, or is it a fixed map?
The co-op on one kart (driver + gunner) is a brilliant design choice. That's the kind of feature that makes people share your game with friends. I'd lean into that in your marketing — "play with a friend on one kart" is immediately understandable and appealing.
One suggestion: if you're not already, consider adding a browser-playable demo. A lot of feedback requests on itch.io go untested because people don't want to download. A web build (even stripped-down) would massively increase your play rate.
Looks like you've got a solid foundation. Good luck with the boss — sounds terrifying!