Migration broke DKIM for personal Gmail + Squarespace domain - how to fix?

I have personal Gmail (Google One NOT Google Workspace) and a custom domain. As soon as I was migrated to SquareSpace I lost authentication and was being bounced or sent to spam. I have contacted Squarespace and Google. Neither has been helpful. Each tells me I'm having a problem with the other. I have searched the web and this forum extensively and not been able to solve the problem.

I was able to add a DNS record to my SquareSpace domain to fix SPF so this is no longer a problem. I wonder why this was not done as part of the migration!

Email sent to my custom domain email address is forwarded to gmail, so no problem there.

Email sent from gmail using send-as is sent and I accept that if you look in the headers you see my underlying gmail address as well as my custom domain email address. That's OK. However, before the migration I had DKIM and now I don't.

Does anyone know if I can fix DKIM and, if so, how?

Traff