Outlook emails to Gmail recipients: Get 421-4.7.28 temporary rate limit failure every other mailing

Hello. I am the volunteer IT director for my church. We send out periodic emails to our congregation with announcements from Microsoft 365 Business (Outlook), in which we have set up an email address for our Bluehost domain name. When we do this, there are generally 160 recipients sent bcc, and about 56 of those are Gmail email addresses. When I took over in February, all email sent to Gmail was being blocked because DKIM and DMARC had not been set up on our domain, which I addressed (SPF was already set up). After that was done, the next mailing like this went out fine. Then, about two weeks later, we started getting this error on all mailing like this with for the Gmail recipients: 421-4.7.28 Gmail has detected an unusual rate of mail originating from your SPF 421-4.7.28 domain [<domain name>.com 35]. To protect our users from 421-4.7.28 spam, mail sent from your domain has been temporarily rate limited. After a few weeks passed, another mailing was received without incident to our Gmail recipients. However, the very next one resulted in the error message above again. I have used every checker I can think of (Mxtoolbox, aboutmy.email, and Gmail Postmaster Tools), and they all see no issues with SPF, DKIM, or DMARC. The only thing left I can think of is that even through we in no way send 5,000 emails a day (none of our emails are marketing emails in any way), Gmail seems to be placing the bulk sender rules on us. If this is the case, I have been able to put an in-body unsubscribe link in our emails, but I don't know if the one-click unsubscribe requirement can be accomplished in Microsoft 365 Business. Just looking to see if anyone else has had a similar situation, and knows of a way to resolve this. Also curious if anyone had any information about the one click unsubscribe availability in Outlook outbound emails. Thanks in advance!