Notice the comment on the story: "I had the exact same problems in the past. For me it helped just to register and immediately cancel a trial of Google Apps for the domain. It’s annoying to have this as a necessary step but at least it was like that done in just a few minutes (after I tried to find a contact email of Google for a much longer time). I tried it because I had other domains on the same server which didn’…
Thanks, may have to pay Google the $5 (or thereabouts?) to host my mail for a month and consider it a cost of freedom from them long term. After 2+ years of hosting my domain myself, it doesn't seem possible to build up enough reputation with them personally.
What this looks like to me is a reputation problem with one or more domains, either in your headers or your message content.
From a domain reputation perspective, you don't generally need to 'build' reputation in the same way you often do with IP reputation; rather, Gmail tends to tag domains with negative reputation only when they've observed unusually bad behavior associated with the domain.
Sometimes this can happen due to domain forgery or spoofing. Sometimes it's because a sender is doing overly aggressive email marketing. Sometimes it's just because your domain shows up in someone else's bulk email.
With full authentication, spoofing is unlikely; I also don't see any indication you're doing email marketing here, and your IP 88.198.5.136 doesn't appear to generate much more than a trickle of volume. So - perhaps the latter? Has anyone included a link to one of your blog posts in an email newsletter, for example?
It would also be helpful to see full headers and content; it's great you're doing SPF/DKIM/DMARC, but they're not going to prevent all issues - there might be something elsewhere in your message that gives us a better idea where the problem is.
Finally, keep in mind that anti-spam systems are highly dynamic, and results can change - sometimes in as little as a few minutes - based on the latest data feeding into the system. If you're lucky, you might see this resolve on its own fairly quickly.