Received your PM. The SPF and DKIM are both good.
I signed up for your forum using a custom domain using iCloud.com and it reported no problems. It immediately went into the inbox as expected. I then changed my email address to an Outlook domain which was fine also. I finally changed my email address to Gmail. That message went into the spam folder.
In Gmail, if you choose "Show Original", it will give you the raw headers and the status of the SPF/DKIM. For the email change notification, it reported:
Original Message
Message ID <614f7db4.1c69fb81.1a599.5cae@mx.google.com>
Created at: Sat, Sep 25, 2021 at 3:51 PM (Delivered after 2 seconds)
From: Our Beloved Ducks Forum <forum@YOURDOMAIN.COM>
To: MYEMAIL@gmail.com
Subject: Please confirm your email address change
SPF: PASS with IP 209.85.220.65 Learn more
DKIM: 'PASS' with domain YOURDOMAIN.COM Learn more
So it's not a SPF/DKIM issue. You're good on that front.
Gmail simply reports:
Why is this message in spam? It is similar to messages that were identified as spam in the past.
So it's not a technical limitation on your setup/config. My guess is the domain itself has received spam complaints in the past. You might want to use Google's Postmaster Tools site...
https://postmaster.google.com
Since you're using Google Workspace, you can actually reach out to Google for assistance.
https://support.google.com/a/table/3247295?hl=en-AU
They should be able to help note why their systems are flagging stuff as spam.