Why use GrapheneOS if you are going to install Google Services anyway? The whole point of Graphene is to have a fully locked down OS that still works as it should. A mobile fortress basically. Installing Google Services defeats the point imo as it opens multiple security holes in the fortress.
May as well just install the stock os. At the end of the day, once Google stops shipping sec updates for your phone, firmware updates stop so that's it really. Graphene cannot give you the firmware updates anyway. And at that point, you have a vulnerable phone. I think graphene os makes more sense if you go all in. Otherwise there is no much point really.