I saw a video with Naomi on Youtube where she uses a separate hotspot that itself has a SIM in it rather than having the SIM in the phone itself. Although this approach does appear more inconvenient than an internal SIM, doesn't it solve some of the issues with SIM privacy if you connect via WiFI to the mobile hotspot (for example, the "SIMO Solis Lite 4G LTE WiFi Mobile Hotspot").
I haven't yet taken the plunge to buy a Pixel phone and install GrapheneOS, but once I understand these issues better, I do plan to.