While rather easy to do, Synology provides so much more “out of the box”. My own NAS ran well on Debian 10 for a few years, until I started getting random disconnects on drives and various other “stability” issues. Synology costs more up front, but after initial configuration it’s more or less just a box in the corner you forget about. Mine just sits there, automatically installing software updates whenever they’re available.
Does your synology utilize BTRFS? If so, what is your experience with it? I'm currently using a FreeNAS box that runs on ZFS that has been very reliable for me over the years. I can't really see giving up the power/reliability of ZFS to move to a synology setup, but I like to keep tabs on BTRFS progress.