For GrapheneOS, it's primarily that it's re-lockable. That's why other unlockable phones aren't supported.

The GrapheneOS install process sets new OS signing keys so you can lock the phone again and get full verified boot. However, most manufacturers haven't implemented this feature.

Is swiftphone its own thing or did you mean shiftphone? I kinda want the shiftphone 8 myself even if they only ship to neighboring countries of mine.

All of these are insecure as hell. Linux phones especially https://madaidans-insecurities.github.io/linux-phones.html

Fairphone also really fucked up: They signed their own OS with the publicly available (!) AOSP test signing keys. These guys really don't know that they're doing, and I would trust their hardware or software whatsoever. And no, installing a custom ROM doesn't solve this. Considering how bad their security practices are, we genuinely have to assume that there are security issues with the device firmware as well.

/e/OS is based on the already insecure LineageOS, and it weakens the security further, so it's not a good option either.

None of the options you mentioned can be compared to GrapheneOS. It's currently the best option if you value your privacy and security. You don't have to give Google money either, since you can just buy a used device, which is also cheaper and more environmentally friendly. Google also makes repairing their devices pretty easy for consumers and even works with iFixit. Here's a Mastodon post I recently saw about that: https://social.linux.pizza/@midtsveen/113630773097519792

I don't think LOS has any privacy/security improvements over the stock android?

(IIRC) it's even worse than stock because you can't lock the bootloader after installation.

Though if your phone isn't getting official updates, it's probably safer with LOS.

I read somewhere that on some xiaomi phones in china you need to request it, https://github.com/melontini/bootloader-unlock-wall-of-shame/blob/main/brands/xiaomi/README.md