The domain and email content always changes (and the domain itself is something random and unfilterable).
The only common thread for all the spam is that the domain it was sent from doesn't match the domain in the "From:" address. The sender's address, the subject of the emails and the content of the message are different, with "via sendgrid.net" the only common link between them.
Wondering if any enterprising HN user was able to beat this spammer's game
https://support.google.com/mail/thread/60339121/how-do-i-block-and-send-all-emails-coming-via-sendgrid-net-to-spam?hl=en
The above is a long thread of messages on the topic, which Google support has not yet been successfully addressed.