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.