One area of failing was amavis which one instance inspects mail, having more than daemon now  seems impossible, some years ago i had four amavis servers but attempts to recreate that left an inconsistent mess sometimes it worked and other time spewed errors by the dozen.
So it was back to improving the single instance.
But $inet_socket_port can have more than one value with = [10024,100xx,etc];
So one instance might suffice.for inbound delivery to amavis.
outbound paths are done with $forward_method and this too becomes = [‘smtp:127.0.0.1:10025’, ‘smtp:127.0.0.1:10027’] ; opposed to the simple defaults in the amavis config hiding in /etc/amavis/conf.d if your debian based like the zoo
That seems (debian stable) to work but it is not guaranteed it might and then again might go back to the first method.
Amavis with it’s perl like configuration is a learn the hard way experience if you want something extra from it.
Don’t forget those semi colons.
This kind of makes sense although it is not guaranteed that the plumbing will go to the right outout but a tolerant spf config allows for that.
I like the one daemon rather than four of them and while not perfect it will do – a discussion on atps will follow so not finished yet..