01
You reply from the Gmail address
Most likely, given it is the address you publish everywhere.
Then none of the records above touch your replies at all. Google signs consumer Gmail under gmail.com and it authenticates fine. The cause would be something else: links, attachments, an image-heavy signature, or simply that the couple has never written to you before.
What it would mean. Rules out the DNS gap as the cause. Points at message content.
02
You reply from an address at your domain
Plausible: your domain has Google mail records, so a mailbox probably exists.
Then the missing DKIM matters a great deal. Your replies would be carrying one authentication signal where they could carry two, and a receiving mail server has less reason to trust them.
What it would mean. Makes DKIM the first fix, and it is a ten minute job.
03
You reply through a Gmail \“send as\” alias
Common, easy to set up years ago and forget, and the worst of the three.
Your message would say it is from your domain while being sent in a way your domain does not vouch for. That is the exact shape spam filters are built to catch, and it would explain a persistent problem that never quite gets bad enough to bounce.
What it would mean. Makes DKIM urgent, and explains why it has gone on this long.
One question settles it: when you reply to a couple, does the message come from the gmail address or from one at farreverafterevents.com? That is the whole diagnosis.