I am experiencing this same issue. My TXT records exceed the 255 character limit given by Shopify's DNS entry settings. Have you found a solution to get a 1024 or 2048 record that is below 255 characters? Thank you

Currently running into this issue as well. Cannot add the TXT record value from the Google Admin console as it exceeds 255 characters. Huge bummer Shopify has not provided clarity on this yet.