Hey, one of the devs is here.
Great feedback, appreciate stuff like this a lot.
Regarding the debuffs, they are intentionally that hard and change your play-feeling a lot, since it's a huge risk - however, I do agree that in the end the black card is too hard to beat. We were playing with this mechanic for a while to make it extreme risky but possible and, I guess, it ended up to be not "possible" enough. x)
About the time for collecting coins, it's also a part of the gameplay: you don't only have to kill all the enemies but also manage to collect everything you got (like, keep one following you and get everything from this wave, otherwise, you'll face more harder waves). :D