I don't think it would be polite to give a bunch of them to cashier

Foreigners always overthink "politeness" issues in Japan... Handling payments, in whatever (legal) form, is their job, they will dutifully do it without any issue. Just try not to do it during busy times, out of consideration for other customers waiting behind you. If you want to donate them, many businesses such as your nearest McDonald's, have donation boxes for various causes, this will be very much appreciated.

I don't think it would be polite to give a bunch of them to cashier

Foreigners always overthink "politeness" issues in Japan... Handling payments, in whatever (legal) form, is their job, they will dutifully do it without any issues. Just try not to do it during busy times, out of consideration for other customers waiting behind you. If you want to donate them, many businesses such as your nearest McDonald's have donation boxes for various causes, this will be very much appreciated.