I have to say I experimented with gentoo back in the day. I liken it to pulling out your toe-nails then trying to put them back in so you can pull them out again. It's a facetious analog but I don't have the patience to read hundreds of lines of instructions to get the stages installed.I'm fine with a bit of a challenge when setting up my system (as I said, I'm currently running Gentoo),
however, your argument about tailering to your own needs I would say is also applicable to Linux (I'd say thats bassically the reason why Distros like Arch and Gentoo exist). Also your comment about how Linux distros are all different I think is true to a degree, but they all are effectivelly the same base (Linux with GNU userspace), the only REAL difference between them I would argue is probably the package manager, everything else can be completely stripped out of a distro and with enough work you could make it almost identical to any other.
While the libs etc might be GNU for MOST it's not for all, the various OS combinations are so varied they might as well not have a common library and utilities from GNU. You're free to locate them anywhere (eg Busybox). You can use different device managers etc.
The key, and major, difference is the init system. Apart from that abomination known as systemd, there's upstart, sysvinit etc. Then there's package managers as you state, which can vary in command structure, ie, pacman or apt-get or the myriad of others.
I think that's what makes *BSDs attractive, as it's the entire OS, not just the kernel with blobs of junk attached to make it an OS. Don't get me wrong I use MX Linux on my laptop exclusively and run Devuan on Arm and iMX devices BUT if you want to swap from Mint to MX the learning curve is quite steep in some ways especially for a novice.
If you're unconcerned about how the system is configured and just want an OS to install and use, then the variants of GNU-Linux like Ubuntu (et al), MX, Mint etc would do just as well as GhostBSD etc.