I use sendgrid as my outgoing smtp relay to avoid ip reputation issues you mention. You still have to configure your dns settings for spf and dkim pointing at their servers instead of yours. Their free tier is 10x the email I’ll ever send so it doesn’t cost anything. There are a few companies in this space with free tiers. It works, but it isnt Gmail level deliverability. I still get spam binned occasionally.

This is signal detection theory combined with an arms race that keeps the problem hard. You cannot block scrapers without blocking people, and you cannot inconvenience bots without also inconveniencing readers. You might figure something clever out temporarily, but eventually this truism will resurface. Excuse me while I solve a few more captchas.

I used stunnel years ago to tunnel both openVPN and SSH traffic and it worked flawlessly. Looks just like https web traffic to dpi software. Beware though, that long open connections can also set off flags, so don’t keep connection’s open permanently.