This is really fun, the scaling is at a surprisingly perfect rate so that you have to be extremely conscious of what upgrades you pick. Overall it's perfectly balanced and really fun to play with a nice learning curve and progression. My high score after a few attempts was 800. I love being able to use reactions as well. This game is super fun and I hope to see a full version in the future.
There are only 3 minor things I would change it would change:
1. A way to rearrange the elements you choose in the hotbar and also be able to save elements for later in your inventory so that you can get a full inventory of every element, just not usable at once. To balance this elements may need to become more uncommon though, and I would maybe even mix them in with the standard loot pool of upgrades.
2. More enemies! The ghosts scaling was fine and still lead to really fun gameplay as enormous hoards of powerful ghosts covering the entire screen move toward you. However, it was a bit confusing at first that I couldn't one shot a ghost with 20 damage upgrades later in the game even though I could with none at the beginning. To avoid this it would be nice if there were some other creatures that appeared as you progressed and all had set health values, speed, and damage.
3. Optimization. I couldn't really tell if all the ghosts were loaded all the time, but I assume they were as at one point there were hundreds upon hundreds of ghosts on my screen, which lagged the game to about 10 fps even when I ran away. Another option to fix this may be to remove collision and just have some checks be used to keep the ghosts from running over each other, but that might not work and would make pathfinding tougher.
Only the last one is something that really needs any attention and the others are more so quality of life. That's just my opinion. Overall I really enjoyed the game, it had a really nice artstyle, memorable music, and super engaging gameplay. I will be playing it more in the future. Looking forward to a full release!