It will be a lot of work, no question about that. Fortunately none of it will be about Maildir, as folks like you have long since ironed out all those issues (thanks!).

It's possible to package qmail in such a way that it's trivial to install and run, supporting many modern features by default. I've done it. Here's a demo: https://youtu.be/Vq6vu9T3vow

But that required a lot of decision-making and a lot of effort by the packager. With notqmail, we hope to make packaging much much easier.