Multiple country questions

Use an ATM card and take out as much local currency as necessary in each place. If you have leftover, use it to pay your hotel bill in each country when you leave. Don't expect taxis to take anything but local cash in whatever country you are in. I wouldn't assume that I could use the credit card for small amounts - entrance fees, restaurants, souvenirs, etc.

Taxis in Frankfurt and Munich take US credit cards. If absolutely necessary, I would use a taxi but have not as yet in these two cities. As for Uber, I avoid them, not an option period.

No Uber in Germany.
The taxis in Frankfurt take credit cards, but it is always a good idea to ask first. Public transportation is cheaper and very easy to use. You will need coins or small bills for local transportation tickets as the ticket machines won't take credit cards. Credit cards for long distance train tickets are accepted though.

No Uber in Budapest. Taxis take Credit Cards and Hungarian Forints; and at less than great exchange rates they will take dollars and euros. Suggest you use Fotaxi from the airport then City Taxi for everything else.

Pull cash in fairly large chunks from ATMs to save on service fees.

I can give you a mailing address for left over Euros and Forints.

Most taxis in Prague take CCs. Or cash of course.

Pulling cash in large chunks depends on how your bank charges you. If your bank charges both a percentage and a fee for an ATM withdrawal, you save money by making fewer withdrawals. If your bank only charges you a percentage only (mine, USAA, charges me one percent with no added fee), you can take any amount and it won't make a difference.

It's never a bad idea to review your banking before a trip. Some of the big banks charge fairly outrageous foreign ATM fees - maybe enough to open a dedicated travel account somewhere else.

Yes, Uber is banned in Germany...good, unlike in SF