Interestingly enough, Amex (Japan) app is also the only banking app I have that is not supported on my (stock xperia) phone anymore, requiring a newer OS version that isn't available on the somewhat old phone. I was thinking to get a Pixel 8 to be able to run a new version of Android, and use Graphene OS - but from what I see above it looks like while that will technically work, it won't be terribly convenient.
(I have a newer phone already, but I keep all my apps on a separate "banking" phone).