To add to that last point, after some time being frustrated with an FP4 and browsing the FP forums: it's not just availability of spare parts to repair shops. There doesn't appear to be a single mention in the forums of a repair shop willing to work on the FP4. There are numerous mentions of repair shops that are unwilling to, and of people being unable to find repair shops. Warranty repair is actually outsourced to a generic warranty support company that appears to operate on standard, replace-everything basis.
Apparently, for the FP4 (reportedly in a departure from some previous devices), all software development is also outsourced. Security patches are often months out of date. Known defects with basic functionality (camera hardware should be fine, but is almost unusable for many tasks because of poor firmware/software making, eg, focusing ridiculously slow; microphone noise and echo reduction has serious problems for calls, etc) go unaddressed, or, in worse cases, are acknowledged as something that won't be fixed, especially for the bizarre security flaw that the manufacturer's official OS is signed by test keys, and the device is set to trust test keys, something that apparently can't be changed, and makes the device one minor leak away from having secure boot entirely broken. This is apparently one of the reasons why GrapheneOS developers refuse to consider supporting it. At the same time, other flaws in the security design make it very easy to brick the device, as you point out, and FP charges for unbricking (the test key problem apparently makes it unsafe for them to release a fix): in one case on the forum, a user bricked the device by correctly following FP's own official instructions for updating the OS manually, and FP refused to unbrick it without charge.
I was astonishingly disappointed by the FP4, to be honest, and by Fairphone in general.