I used iPhone6s plus before, but after less than two years, it became difficult to use. It may be that I have been playing games with it for a while, and I have been constantly updating the software, making it unusable. Now I I bought a new phone, and I want it to accompany me for three years. In addition, I also updated the system to iOS14. How can I protect my iPhone?
You can't really protect your iPhone from Apple's intentional obsolescence and continual upgrade path where the combination of software upgrades and higher hardware needs from games to take advantage of the newer processor and the newer graphics core processor in the new iPhone will make older iPhones obsolete, which then forces you to upgrade your phone on a bi-annually basis! That doesn't take account of the battery age which also can slow it down from peak performance.
The iPhone is like a normal computer. Once you buy it, it is obsolete because next year, Apple will have a new A series chip or the M series chip for Macs that are faster than the older chips.
There's nothing to protect your investments other than being aware that your iPhone 12 will be superceded by 13, then 14, then 15 with better, faster and more capable chips and graphics. Then new games will take advantage of the newer chips and more available RAM. Which is why PC gaming can get very expensive. You have to also keep up with newer graphics and newer CPUs just to play those newer games smoothly.
The person who doesn't game, doesn't do a lot of CPU and graphics intensive apps can still use the iPhone 6s Plus today as if it's just as fast as in the past. However, once you start pushing it, then the A9 chip in the 6s Plus, which is also in my iPhone SE 2016 is sluggish even with a somewhat new battery from Apple with 94% life left. Which is why I got a new SE 2020 to replace it. Now running the same intensive apps that made my SE 2016 sluggish is smooth on the SE 2020. There's no comparison between an A13 chip vs an A9 chip and there's no comparison between your iPhone 12, which is an A14 chip versus the A9 in your iPhone 6s Plus.
My parents who bought my old iPhone SE 2016 is delighted to have a super fast phone, especially coming off an aging iPhone 4S which died recently.