Thank you for your looooong but really pleasant to read review, with both positive and negative points (perfection doesn't exist). The tax system was very inspired by the roguelite : The binding of Isaac, where you can trade health points for powerful upgrades, that's basically the same idea here, you can put you in danger but you get access to more fire power, classic risk/reward mechanic.
For the screen shake you are totally right, the problem is that each shake stack with the others, leading to an earthquake when the shot rate and multiple shots are acquired.
if you have reached 200000+ gold, you had a taste of real power :) I didn't had the time to make a new enemies archetypes with 4HP or more to counter the OP endgame DPS, so passing wave 30 I spawn 18 mobs per second, but it's clearly far from enough to reach the wall...
Spoiler : the knights doesn't even exists. I didn't had time to code them... So 9 999 999 gold for nothing is pretty expansive I guess : )