Can you elaborate on MicroG needing root? To my understanding that is only required on ROMs that don’t require Sig. Spoofing, and Calyx does support it, specifically and only for MicroG.
I'm not entirely sure if all of microG needs to run as root, but I'm pretty sure that some parts do. Nonetheless, microG runs in the priv_app SELinux domain instead of untrusted_app, reducing the isolation and granting it more access to sensitive APIs. Sandboxed Google Play on GrapheneOS on the other hand is a normal application that can be installed and uninstalled by the user, running in the untrusted_app domain. It is tightly controlled by the Android permission mechanism, and doesn't have any permissions by default.