In case you missed it and end up going there without trying to open it -
McDonalds have helpfully decided that version 9 of Android is no longer supported by their app for "security reasons." Making their app more secure than my banking ones apparently?
Yes you're device is going to be old (mines a Note 8) to have this issue. But why would I change phones when this one is fine?!
And while I could install a custom build of 12-13 this would likely stop my banking apps from working.

Installing custom ROMs isn't going to stop your banking apps from working.
I have banks with basically all the banks in NZ. All their apps work fine on my phones that are rooted and running custom ROMs. You just need to know what to do to hide root from these apps, same goes for spoofing location for Z Sharetank or whatever. There are guides out there.