As someone who spent some time on the topic (result), it’s not that every new app is adaptive. Even if someone uses the nice new widgets of libadwaita (or previously libhandy (GTK3)), that app is not necessary running well on mobile if width-reqests demand a higher minimal width or content is just too wide.
The same is true for QtQuick Components or Kirigami, which are the equivalent for adaptive Qt apps.
That said, yes, many new apps developed with these technologies work fine OOTB without the developer even knowing; and if they are too wide or tall, fixing that is usually rather simple and not a full rewrite/redesign.
Glad that works for you! With my bank (comdirect.de) I can use a mobile website, and if I were to use something AOSP- or Halium-based, I could also use their PhotoTAN app, which, as the name implies, needs a working camera in Waydroid (on my OP6 with pmOS, the cameras work via libcamera, but not in Waydroid), so I have a small gadget for all these TANs.
My main worry with the “let’s just use Play Store/Aurora store and the run that apk”-approach is that it does not really send a visible signal to banks that they need to keep considering customers that don’t use Android proper.
It also always means that the next update (e.g., after some consultancy or some audit happened) may not work any more, meaning, access may be revoked at any time. Complaining to customer service or in Play Store reviews may have an effect, but it will still hurt. I think I would feel a tad safer if a banking app lived on FDroid… but sill.
I hope this gets my point across.