Strange, I’ve successfully been updating a project in unity 2018.4 LTS for mobile for years ( last update a week ago) , with minimal problems and never been forced into updating to a newer Unity version. I have had to make some adjustments and modifications, but that is mostly in iOS plist and for a time I had to add min/req sdk level by hand to the android manifest.
Perhaps i’m fortunate enough that the project just isn’t hitting these pain points? It is still based around APK which I think keeps things a bit simpler than AAB, but I’m not sure that alone would explain it. Its relatively simple project, but still uses a number of 3rd party plugins and deals with various social sharing features. Perhaps there are some specific features like ads or IAP that require more frequent Unity version updates, though I somewhat doubt that occurs 4 times a year?
There are certainly some pain points such as Unity not outputting the symbols zip in the required format for Gooogle Play, but that is easily fixed by hand and only takes a few minutes each time. I do regularly have to update stuff for both Google and Apple when submitting a build, such as license agreements, data usage, privacy etc, but none of that is dependent upon Unity version.
Perhaps if you outlined specifically the problems with builds that necessitate requiring a newer Unity version the community could help with alternative suggestions to ease the process.
Sadly though, for iOS at least, this ease of using an older version of Unity appears to be coming to an end. The new Apple requirements for Privacy manifest ( see this thread ) would be difficult if not impossible to get around.