Strange, there was "post-fs" boot stage "happens after the most partitions are mounted, except /data". Modules stored in /cache/magisk_mount/system.Pretty much everything Magisk related is stored in /data/adb. So, having a Magisk module that doesn't touch /data is going to be quite difficult (read: doesn't work like that)...
But it`s gone now. Is it because google killed /cache partition on A/B devices?
My Galaxy A50 still got /cache!