Is a Chinese phone's stock OS better than GrapheneOS for my threat model?

How? You should not make the Huawai to a dumphone that you do not use. You should still use the Huawai as a main phone, but load sensitive tasks off to the GOS phone. If they pull you over hand your huawai phone that you actually use and keep the GOS hidden (in your underpants or somewhere they do not want to look).