If you were to set up your own SMTP server and try to deliver mail, you essentially cannot reliably email any of the larger providers, because they’ve taken steps to mitigate spam and issues which also makes it impossible to handle your own email anymore, even if the intent wasn’t explicitly to break self-hosting.
But this isn’t true either? I can easily spin up a SMTP server on a homelab, create an MX record, and email my friends with Gmail accounts as if I was emailing from my Protonmail or Gmail account.
I appreciate you acknowledging your bias against central providers, but to be honest I think it’s leading to some incorrect conclusions. This discussion is also kind of getting derailed, but I’d be happy to continue debating about it.
Not quite sure what you’re looking for, but I’ve been using avistaz.to as a private tracker