Best reason to use Void is to have a fully working musl-based distro in no times, and easy to use. However, last time I tried (probably monthes/a year ago now), a 100% musl distro was painful in some cases (eg. awful/inexistent javascript support in firefox).

As you said, the only viable package manager for portability is pkgsrc. And as it works basically everywhere, you can pick any distro. I'd recommend a stable one so your core doesn't change much (like debian or fedora), and "build" on top of that.

I used to do that myself (and still do it !). I wrote my own package manager (pm(1)) which I use as a complement of whatever is the "default" package manager on the distro I use (currently successfully tested on crux, openbsd, debian and ubuntu). It's not "portable" though, as it's tailored to my particular needs, but you can totally replace that with pkgsrc IMO.