I have been running my own mail server with similar requirements for 20 years now.

I empathize that getting flagged by major providers is the most worrisome part.

Yet, it's not as bad as it was in the years 2012-2015 when SPF, dkim and dmarc strated becoming mandatory.

I maintain my outbound server against all odds, mostly because I think it's very important that independent providers can still exist.