One of the reasons why I would buy a repairable phone is it’s reliability in the long run. For instance, my previous phone, a Nokia, had a broken USB-C port. Replacing it would be pretty hard to do myself, or expensive to let somebody else do it. And that is if you can find replacement parts at all. One of the main benefits of a phone like the Fairphone, is that I can just order a new port from the manufacturer for a low price, without any unexpected costs, and replace it in 15 minutes. I still have some photo’s on that phone because they were not automatically backed up. The Nokia also was pretty unusable even when it worked, because the software was borderline criminally bad. However, the bootloader was locked so I couldn’t change it.

buying a second ahnd flagship is also a great way to save a bit on the environment, but it’s won’t be as reliable, the condition of the battery will probably be worse and you’ll have to watch out that you don’t buy a phone that doesn’t get any updates anymore (or at least has a unlocked bootloader if you’re willing to flash a custom ROM)