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.

When the fine for littering and the cost of repair or recycling is higher than what you can recoup from this sort of lost property, it’s a win win for the police.