Whatever the case for departure, I would say that if the ip-address and/or hostname were an issue, that they could still be included in the email perhaps with some massaging.

For example instead of the ip address being four groups of information separated by three periods that the period have a space before and aftewr it and any link coded into the numbers just be removed or similar. Or perhaps (haven't looked at the detail in a while) some additional htmlencoding be applied...

Unfortunately there could still be some other reason that Gmail (even when using the g-suite portion) is flagging the email. We haven't even really talked about the settings that have been applied... smtp? Smtpauth? Whatever the new Gmail server option is? All/each of these provides different information to the servers along the way. Last time I looked at using the g suite for a domain, there seemed like a bunch of things needed to be done to have the email come/go through their servers related to dns records. Sounds like those may have all be addressed, but never hurts to ask or look again from the perspective of trust by the server(s).

Have you tried setting up an email address that would be used for sending such messages but perhaps not used as the recipient as well? Wondering if part of the issue is the receiver is the sender type thing...

Overall, doesn't surprise me that messages with more detail that a basic shopper gets when making a purchase get "treated" differently.