GrapheneOS, like AOSP, of course offers high quality bluetooth codecs. Several actually. APTX HD, LDAC, and so on.
The situation is complicated though.
LDAC, which is used by the sony headsets, for example is set to balanced quality as a standard. In theory that should be enough. However, devices manufacturers decide what 'balanced' means. So for the Pixels if I remember correctly, this is around 300something kbps.
If you want to have the full 990 kbps for LDAC you need to set that in the developer settings.