The most important part of Graphene for most users isn't the super security - it's that it fights for the user, not for some corporation.

Yeah, but I thing the best security is very important to the GrapheneOS developers, so I don't think they will ever compromise on MTE (except pre-Pixel 8) and a secure enclave. Of course, someone could fork GrapheneOS to support phones that don't fulfill the requirement. Also, I think fighting for the user is also true for e.g. LineageOS. As far as I know they are also a non-profit.

It is. But you can get almost all of the benefits on hardware without that. Someone has to port it, though - Graphene won't. Yes, it's also true for LineageOS and you can use that too.

No, you can't obtain almost all of the benefits of GrapheneOS on other hardware. Many of the benefits depend on having current privacy and security patches along with hardware-based security features.

GrapheneOS is a privacy project making major privacy improvements. Privacy depends on security so that's why it improves that too.