In the bank could there be a button or setting to take the interest into your cash pool not add to your account? and i still think that you shouldn't have to collect interest it should be automatic.
This game isn't very idle for an idle game, you have to work too much to get money or do anything.
when you upgrade your units the upgrade goes straight to "MAX" maybe you could put add levels of upgrade onto them other than just the 9 for every unit. perhaps each spell could be upgraded rather than just getting them activated.
It was going to be Whack A Mole, but apparently moles don't live at the South Pole :D