As a "small fry" myself, the hoops I've had to jump through in order for Gmail to accept legitimate mail from my mail server are rather large. Like the op, legit email appears in the trash of some recipients for goodness knows why. Like OP I've spf, dkim, & have never been on rbl. I (& services on my machine) are the only senders, and the recipients of those machine messages want them (about 10ppl in total & email have no marketing content). I communicate on port 25 using tls where possible.

It's only ever Gmail that sends mail to spam. Every test I run on that email marks it as clean (generally spam assassin)

Gmail is certainly NOT a part of the open email system you mention, but is a constant thorn in open communication.

Ironically, I use Gapps for one domain and because of my spf settings with "include:_ghs.google.com" I get every cat and his dog trying to send as users from my domain, which thankfully end up in that domain's gapps spam. (hint: let us use geo located includes, like _ghs-us.google.com or _ghs-au.google.com, so that there's a smaller include list!)