I wish I could simply run 3 year old apps. In many cases I am not allowed, especially the bank ones, which are the ones that I cannot run in the background as the OS kills them.
Bank apps seem to be a bottleneck in many aspects of phone development.
Are you not able to use your bank's mobile or desktop website in your phone's browser? I don't think I've ever installed a banking app. I also use a credit union though, maybe they're less incentivized to force people onto their apps.
My Canadian bank occasionally will block a bank transfer if done from my computer. I have called customer support, and they can't even tell why the transfer is being blocked. Possibly because there is some opaque ML algorithm at the backend.
Customer support has requested that I use the mobile app, and that usually allows the transfer.
Sorry if this is obvious, but have you tried the options in Settings to avoid the OS killing certain apps? On my Pixel it's Settings > Apps > App Battery Usage > (choose app) > Unrestricted
Turning off app updates would have the same impact if you're right about the increasing memory requirements.