They will be able to do banking at least once the legislators tear down the walled gardens in a sensible way. Are the security benefits from the Appstore/Playstore real or security theatre?

I'm pretty sure that, if there are security benefits, they have been artificially tied to the use of the company's distribution method, that coincidentally really needs to be sending usage statistics, monitoring, etc. Surely there exist no conflicts of interest to be found.

Looks like the Swedish bankid at the very least actually does work on sailfish[1]!

Not sure about equivalent apps for other regions, but I don't see why they shouldn't work.

[1] https://forum.sailfishos.org/t/swedish-bank-id-swish/11781/3

Afaik there is an android compatibility layer but I don't know if it allows banking apps to works

You are allowed to have more than one cell phone.

If you use it without compatibility layer it's probably on the same level as a kaios phone. There is a lot of slop on the sailfish store.