I guess I absolutely need the play store to get BankID on the phone- so I’ll try that now with my Pixel 7.
The only issue I had on GrapheneOS was that I had to play with the location permissions a bit when I wanted to copy the BankID to GrapheneOS from another phone (I've got some pictures of that in this blog post: https://www.jonashietala.se/blog/2025/08/28/ill_only_buy_dev...).
All other Swedish bank accounts I've tried have also worked great (including Swish).