The phone app is very useful for those functions - especially if the alternative is going into a bank and queuing.

the bank's web portal should also work for this.

if it refuses to open on your phone, try firefox and enable desktop mode for that tab

Iiuc Open Source Android has to sandbox Google Play for banking apps to work

that's not necessary but also not enough for it. the bank apps specifically check if you run a google/apple approved, unmodified operating system.

today it is not possible to pay with an open source operating system. somewhat of an exception is grapheneos, but even if you just build it from source for yourself, the bank's app will refuse operation.

and on top of that, google services are required too if you want to pay with your phone in shops.

so this is not something open source os maintainers can solve. it is a result of google engaging in illegal practices which are not prosecuted.

building an alternative to Google and Apple is a high priority

Technically... afaik most banks in the EU also support current non-android huawei phones, which have their own implementation of the play integrity service to lock you into the factory os. but huaweis are not better, probably worse.