I played a full run with the witch character last time I touched this game so I tried the other characters this time, starting with the bomb elf. Her main attack is interesting but feels pretty lackluster, it requires a lot more effort than the witch's spam fireballs without much reward. This is not only frustrating but makes the character feel unfun. You could increase the explosion radius and make the bombs deal a lot more damage when hitting enemies near the center of the explosion, as it would still encourage players to have good timing while also bringing up the overall power of the character. There should also be some sort of indicator for your bomb charges, I recommend something like circles representing your bombs around either the cursor or the character sprite.
The cyclops character is just boring, it's the most passive gameplay imaginable. Maybe it's entertaining to VS lovers but I don't see it. I would give something for the player to actually do on top of having the drones, like charging up the drones and having them fire a beam in the direction the player is aiming, then putting the attack on a long cooldown.
The blonde character has an interesting mechanic and it does scale fairly well, not many complaints there. Just make it more obvious you're losing health when you have the damage aura.
The enemy variety is nice but the environment is lacking in both visual and gameplay substance. It would be easy to make some tile variations to spice up the floor, e.g., cracked and mossy tiles. There's only a handful of props with fixed spawns, maybe make more stuff periodically spawn in. I still think your choice of VFX style is poor and doesn't fit the game but you don't seem interested in changing it, so I will leave it be.
I found a bug: pausing and unpausing during the upgrade screen will cause the game to return to x1 timescale while the player is still selecting an upgrade.