Thank You - I spent a couple of days searching many pages all stating the old process for adding an alias; some even from 2024 after App Passwords 'as was' were removed, after the Squarespace move and sadly still listed on Google Help Pages.
This is the first thread I have found that actually is correct and worked as listed.
My Squarespace Domain migration also migrated my email aliases, pointing back to my Gmail address so I have not had any issues that some appear to have had.
Thanks again.