The concept is cool but I think it is more sustainable to have a high end "normal" android phone. The Fairphone ships with a lowend chip that is already older. It may be usable now but it won't be in a few years apps and OS will be more ressource-intensive. A high-end chip of a "normal" device (8gen1/2) is way more powerful and will surely be enough for the next 5 years. The SD750G will propably not.
Not to mention that the software updates will be more reliable on big brand phones.
Hard to say for sure, keep in mind that this is a degoogled by default android, meaning that depending on how much Google services chug (and oh boy, do they chug) the phone may hold up with no problem at all.
While it is a computer, my personal Lenovo T60 from (i think) 2007 is still my favorite machine to browse the internet on and read articles. Once I librebooted it and installed debian, it ran with no problems at all and is very functional till this day