This is a pretty well made game for a game jam! It is very fun and enjoyed the gameplay. However, the most jarring problem would be the difficulty! I could mostly reach wave 5. its very hard to get the upgrades to survive when i have to think on using my money to pay taxes too! It is fun but I do agree that a rework of the difficulty curve could make this game more fun!
thanks, I think the difficulty is not the problem, I even find the game way too easy.
When I watch your gameplay, the first thing I see is that you dont pay the first 2 taxes (the cheapest of all) and you spend all your money on non DPS upgrades...
I think the low price of certain upgrades force some player like you to spend there money on it, cause you want to buy something at all cost, and you dont save your money for powerfull upgrade.
I think its a pure design problem not a difficulty problem, the things i can do to resolve it : nudge the player to buy a 2nd archer, damage upgrade and shot rate upgrade the sooner.
The game is only difficult if you make the bad choices, Its my job to force you to make the right ones.