Before I dive into my critiques, just know that it's because I think this game works well that I spent a while playing it and thinking about what could have been even better! I especially liked how enemies have different speeds, and some "enemies" (the golden ones) should be dodged rather than intercepted. It gave a lot more depth than just "turn towards whatever's closest to you"! I also appreciate the shop polish where text would zoom in instead of just appearing when you mouse over. Everything else is clean and fun in general, too!
The first thing that felt a little off to me, was that you had to unlock the enemy that "you should really upgrade your damage for these" as a requirement to actually upgrade your damage at all. It felt a bit like I was being forced to point the proverbial gun at my foot, and made me feel like I needed to save up a lot to unlock the enemy and those upgrades in one go. The same was true for the spikey enemy, although there was a set of HP upgrades in front, so this is slightly better. I think these should have been reversed, perhaps--gate the damage upgrades behind the spikers and gate the bigger HP upgrades behind the tanks. There could be some initial HP upgrades (as there already are) to encourage the HP -> spiker -> DMG -> tank path, but still allow for players to unlock enemies quickly if they just want to SEND IT.
The Tank needs a little balancing, I think--with no damage upgrades, it felt like it had as much health as the boss? I expected 2-3 hits to kill them initially, not 4-5, and 1 after fully upgrading damage as requested.
Regen is a great idea for an upgrade, but it feels a bit OP currently. It might need a bit of tuning to continue (as it effectively does now) to make mistakes feel less punishing, but not start being used as a mechanic to just ignore many of the little triangles. I started doing that closer to the end and it felt bad letting things hit me, but my HP was staying full still.
Knockback is also a fun mechanic, but too extreme in my opinion. Especially after the knockback increase upgrade, which i shouldn't have purchased, I kept wondering why bosses kept spawning... but then I realized it was just the same boss coming back after like 15 seconds! I wondered why I had 1-shot a boss, and then realized it was the one I had been hitting for a while.
The economy also probably needs some tuning--I bought all of the first set of money upgrades (before the "self" enemy) after buying the cheap (sub-$50) enemy types and hp/damage upgrades. At that point I could basically just farm money until the $1000 threshold to buy the "WIN" upgrade.
Major nitpick: The BUY text in the shop can also probably go away once you've followed the instruction! And the prices increasing after buying the last one doesn't make a lot of sense--they could just say MAX or something to make it clear that they're not for purchase anymore.
Again, amazing game though. I don't play many of the games I look at for very long, or spend time thinking about feedback unless I enjoyed it, so great work!