I own an iMac 2017 and an iPhone SE 1st generation. Big Sur and iOS 14 made me finally realize that what Apple is trying to do is to turn the computer into an appliance, at least within the reach of their "eco-system", and that Apple is openly, but implicitly hostile towards people like us, who want a computer, probably with the goal of making us drop their platforms as soon as possible.
This means that, unfortunately, we'd have to comply sooner or later. Naturally, I'd want to keep using my devices for as long as it is possible for me. Therefore, my strategy is as follows:
1. No software updates.
2. Really, no software updates. Even the "important" ones. Even if the vendor of a software uses scare tactics or begs you. Even if it's a supra-security update that makes the evil haxor go away. Even if the update adds some new feature that you've been dreaming of for the past 10 years.
On a side note, if you still use iOS 13 or earlier, consider yourself lucky. I wish that there was an easy way to go back to iOS 13.
3. The software should never be updated. This applies, obviously, to third-party software as well as the operating system and the software provided by Apple.
On a side note, keep in mind that the software will find a way to update itself if it has the ability to do so, ignoring any settings that you may have set. Therefore, this ability has to be physically removed. In practice, I deleted the softwareupdated binary from the system, Microsoft Auto Update binaries, etc.
4. Unfortunately, there is software that will "refuse" to work if not updated for some time, particularly any software that has "cloud" in its name, or is dependent on third-party APIs in order to function.
When the number of applications that I won't be able to use will be high enough for me not to notice, I will format the Mac OS volume and install some Linux distribution there. I will use a Windows VM for applications that are not available on Linux. Naturally, I'd keep my Bootcamp volume for gaming. Considering Mac-only applications, well, it'll be goodbye then, I'd have to invest time into finding Linux or Windows counterparts and learning how to use them.
5. Speaking about my iPhone, it is likely that it will break before I'd have any problems related to outdated iOS. When this happens, I'd buy an Android phone with some decent hardware, and install a customizable Android build on it.