So Quick point to make this bug is useless on pretty much any category that doesn't require a fresh account. Regardless I did find it, it seems undocumented and actually Speedrun viable only problem is its lag based. I don't know if lag based bugs would even be allowed in speedruns so I'm curious.

Way the bug works: You can buy tower upgrades you don't have the EXP for (potentially even ones you dont have the money for)

Explanation: I was doing a coop Bloons stream and the players around me had horrible ping as the game would constantly pause or revert however because of this I somehow managed to buy an upgrade I didn't and still don't have unlocked due to lack of exp with only one other player in the coop game. Time stamp for the bug starting is 2:56:30 and this is my stream vod here. Its the T4 alchemist I bought if you keep watching you will even see that I notice the bug during the game.