Mount /system on Android 10 with Magisk
Your second command is correct, but the virtual block device is read only. IIRC, this is caused by the a/b mapping virtualisations.

Referring to https://android.stackexchange.com/a/158890/171244, you can see that specifying the correct block device will help. Try:

adb shell
su -c mount -wo remount /dev/block/bootdevice/by-name/system$(getprop ro.boot.slot_suffix) /system


This will switch from the virtual mapper (/dev/root) which is read-only, to the actual partition block device, making it writable