Migrating from macOS Server Mail service to Postfix/Dovecot

I have been searching for days and days on the web to try to find some sort of guidance on this. Searches either send me to people wanting to set up a local SMTP or postfix server for themselves on their laptop ie. nothing to do with macOS server or migrating from macOS server to open source command line tools, or it's a news article about Apple deprecating many features from macOS server with no links to send me in the right direction except to the dovecot homepage etc.

Basically, I'm happy to have Apple deprecate the services as they can be problematic with the custom way Apple has set them up. We all constantly get prompts to input our password again and again when the password is already correct. It's prompted me to be up for the challenge of going all in on postfix and dovecot vanilla installs, hoping that I can make the whole mail server thing be far more robust, and maybe I can add in webmail and things like that at one point.

I've turned off the mail service in macOS server, used brew to install dovecot and all went fine, installed openssl as well. But from there, any guide I've found seems to suggest that's all I need to do. I've started the service and had it start at startup according to the brew instructions. But nothing is working or on, or connected at all. I've installed postfixadmin (web interface for creating users and domains) but those settings aren't automatically connected to dovecot/postfix and that also gives an error that php-imap isn't installed which brew won't install any more with php 7.1 because of a lack of stability. So it's all a mess. I'm sure I could plug away but in order to trial and error this, it could be days, which means days of my business and users being without mail. So I gave up and turned on the mail service in macOS server again, once I realised this was not going to be a 2-4 hour job but much longer as I try to troubleshoot and experiment. Again, normally I'm happy to do that, but with something this vital and important, I can't afford to. I need a clear and distinct and specific guide to this specific situation, but cannot find one anywhere.

I've found generic install guides for linux or ubuntu but they don't seem right for macOS, and they don't take into account the fact that I already have a working install of postfix and dovecot via macOS server. All the mail data from the server is dovecot already and it would seem to make sense to just transfer that over to the vanilla install, but have no idea how. I'm also open to ditching all the user data (as I've had the 20+ users move their mail data to their local macs) and setting up a completely fresh install. But the how to do it is really not clear for what I would think is now a fairly common situation - someone who has been running macOS server mail service for years who now is being forced to shut it down and go the whole DIY route with command line tools.

This should be:

A. Easier than it seems to me right now. If you've never done it before, you just don't have the experience and it seems to be overwhelming, which surely can't be the case for someone that knows what they're doing.

B. Something that Apple should have provided a guide to. There is a PDF guide that Apple wrote that I've downloaded, that has instructions for migrating EVERY other deprecated service EXCEPT for mail. Which is mind boggling to me. They just skipped over it, don't even mention the mail service.

C. A common enough situaton that someone else would have written a guide to (along with revenue generating ads on their article) because with Apple deprecating it, I would have thought a whole bunch of people would be trying to do what I'm doing. So surely someone has already written a guide like this, but I just can't find it. So a link to it would be handy!

Any thoughts, or ideas to point me in the right direction?

Thank you!