I'm in that boat. I'm looking over at that Synology unit sitting in the corner of my living room, knowing it'll be the last of its kind to live here, and wondering what its replacement will look like. FreeBSD's been good to me and it might be time to reintroduce myself to it.

Doesn't TrueNAS (Linux version) come with ZFS?

TrueNAS Scale, which is the Linux variant, does indeed come with ZFS.

The comapny behind it, iXsystems, pays for ZFS developers as well.

There are some Linux distros with ZFS as a near first class citizen. NixOS is one, I believe Alpine is another.

I looked long and hard at FreeBSD and eventually went with Gentoo on ZFS, as I was familiar with Gentoo and ZFS wasn't hard to add.

Ubuntu too.

Proxmox is Debian + ZFS. Works great.

Fwiw, I'm running a NAS on btrfs (on top of mdadm raid as I don't fully trust the btrfs raid, and the recovery tools seem worse). It seems to be working well so far.

Being able to do snapshots easily is really nice. I have a script that makes hourly snapshots and keeps the N latest, which protects me against a bunch of pebkac errors

I do periodic backups to non-btrfs storage though. I need backups anyway so it seemed like an easy way to de-risk

Go for it. I made the switch ~10 years ago and didn't regret it at all. First-class, rock solid ZFS integration. Saved my data on more than one occasion.

FreeBSD is extremely simple to keep up to date, including third party packages.

Unless you have some very specific reason to use Linux, I would go with FreeBSD.