If Wero is going to be similar to iDEAL, which it is based on, the payment flow will be through your bank's app. Heck, even the Wero app description says so:

The Wero app is available only for account holders from the German bank Postbank and the French bank La Banque Postale.

Are you a customer of another Wero-enabled bank? If so, you can easily use Wero within your banking app.

https://play.google.com/store/apps/details?id=eu.epicompany....

I have seen quite some people downloading their banking apps through the Aurora store (admittedly, it's a frontend for the Play Store, but you don't need an account or the app). Or you can use something like APKMirror and verify that they have the correct signing keys.

Of course, if they need Play Services or Play Integrity basic, then you need sandboxed Google Play or microG. If your bank does remote attestation through Play Integrity, then it is game over, but it is worth contacting your bank. E.g. some banks have added the GrapheneOS signing keys and added support for AOSP remote attestation.