grapheneos has a hard requirement for the vendor to provide software support for 6+ years (iirc), so i expect the updates will be better even for stock users
however this might be only for their new Motorola Signature line of flagships...
Our official requirement is 5 years of support meeting our standards but it will be raised to 7 at some point. The Motorola Signature (2026) already has 7 years of support but it's future devices which are going to meet all our requirements and provide official GrapheneOS support.
however this might be only for their new Motorola Signature line of flagships...