Like others said, banking needs licensing and licensing costs money. If you already have a bank account, you already trust one party. Ask them if they roll their own app-payment or are already partnered with a service. That way, you can avoid google/Apple and minimize spreading the trust to other parties. My bank cooperates with Fidesmo, for example. Fidesmo then sells wearables with nfc-pay.

Doesn't exist. I was also surprised that this was not NFC-related tech, but google/apple-specific.

I guess it makes sense - you need some kind of authority to validate/take blame for transactions (outside of the hot mess of block chain), I had just always presumed that the tech didn't need much to work.

You are in germany, so you will need to wait for GNU Taler for that:

https://taler.net/

Phone case with space for a credit card under the back. https://www.amazon.co.uk/Spigen-Armor-Compatible-Google-Pixel-Black/dp/B0BZ5PBKXP

I mean there's GNU-Taler but it hasn't acheived mass-adoption Unless your stores has a cryptocurrency option, you're better off with cash

Private payments with a wallet on you phone is something the digital Euro is supposed to offer, if it ever arrives as designed.

Paypal, Volksbank VR Pay/ Sparkasse or any other supported bank maybe. Privacy friendly? Good luck….

I think I heard cake wallet is able to do NFC payments for places that accept Monero.

Did you look into garmin watches? GOS has no pay function.