Deleting that "dummy account" is probably not a good idea in the new T2 era. The initial administrator account is used in validation with security settings authentication in the recovery mode. There is a way with APFS that this account can be 'shared' across those two OS instances.... Two other suggestions: I have also found it more reliable to first create a dummy user on the new machine, complete all updates, check/repair the boot disk and basically confirm it is stable (ideally, make a CCC backup of this stable installation, too). Only then launch Migration Assistant and migrate from the good CCC backup. (The dummy user account can be deleted later).
A better, more stable baseline is to create an "admin" account on each Mac. You do not migrate this one. It is always the first one (so it always gets UID 501), and you don't really keep have to keep much there. Just use it to install software for the machine—upgrades, security updates, 3rd-party updates/upgrades (that don't have some system auto update daemon), etc.
If you do this the same way each time for the same set of accounts, the UIDs on external (and shared) drives will all match up to the same consistent UID/username layout. If you put in a dummy account and move all of the accounts over from another machine, what you'll likely get is UID drift (unless the Migration Assistant has gotten smarter - I haven't tested if it has gotten smarter about matching exact UID/unsername pairs up to each other and treating them as equivalent equals).