Lovely little game, very nicely done. Graphics look nice and music is nicely fitting.
In terms of balance, the projectile is very overpowered. Especially combining it with damage upgrades (like I did). In fact, it was so powerful that I never even saw what kind of attack the mage type enemies do, because I would kill them before they even did anything. To balance this better, I'd make the mages attack from a longer distance, so you actually have to consider them a threat.
Adding a dash or other movement based abilities would be a nice way to make the combat more dynamic and varied. You could change up the terrain to make movement more interesting, like adding pillars or spiked pits so you can't just move anywhere you want.
The upgrades were a nice idea, but I would try to steer away from just changing around numbers. As far as upgrades go, those are by far the most boring kind. Less passive and more active would be my suggestion. Add more things to actually do with the upgrades, more abilities, more weapon types, etc.
Abilities could be like, dash, blink, teleport, turn invisible for a few seconds, block the next hit, do a spear jab, throw a net, throw your weapon (which you then have to retrieve), dual wield weapons, basically anything that changes up the gameplay.
Best of luck!