The choice of only supporting Pixels comes from GrapheneOS's side, not Fairphone. Fairphone got some great ROMs support, and even have an official partnership with one of them (e/OS).
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With unofficial parts you can get anything, from very good parts to outright dangerous ones (especially batteries).
The problem with IPhone is their association system (which is illegal in the EU BTW). Understandable with those dangerous part on the market, but far too overcharging. They could just warn you during boot or something like that.
As for the software wall, it is where Fairphones shines. Even when the official support ends, the custom ROMs keeps updating for a while. IPhones are great on that aspect too, Samsungs are OK, but can't say for other brands.