FreeBSD user here too <3 Mainly because I think Linux is way too aligned (and developed by) big tech these days.
We have Netflix and Sun influence but the former is not really putting its stamp on it and the latter no longer exists (and evil Oracle has zero interest of course)
I prefer the OS aligned with users like me not the big cloud boys.
FreeBSD is kinda declarative. A lot of it is (or can be) configured in a text file called rc.conf
https://man.freebsd.org/cgi/man.cgi?rc.conf
It's not as completely declarative as Nix but it was never intended to be.
What about declarative listing of installed packages? One of the neat things about NixOS is that I know exactly what packages I've manually installed since they're specified in my config file.
The other big difference I notice between NixOS and e.g. Debian is that on NixOS I only have to specify the changes from the defaults. On e.g. Debian I usually have to change config files that already exist. Then when the package updates its default config I end up having to review all the changes to the config, even when they have nothing to do with what I've explicitly configured.
You should really give it a try.
I would love to try FreeBSD but I'm too addicted to NixOS... If there was a way to have a NixOS-like declarative BSD system I'd give it a serious try.