Yes it is uncommon, as people have mentioned it is a Wireguard wrapper. Try just using Wireguard and troubleshoot from there. I would want to know what that 'simple' solution is failing. Your workaround could have similar issues.

I am also using tailscale via headscale, and my connection has good. Only when the internet where my home server is down is the connection not available, and there isn't much I can do about that.

Never had an issue with Tailscale and have been using it for years.

You don't have to be on the NAS' physical network to run some tests, if you can SSH into it, that's enough to run some speed tests and latency tests to reference servers (e.g. Google, netflix, ...), while doing the same from your PC simultaneously. Whenever you encounter "instabilities" you should be able to see which side of the connection is struggling more than usual. You should also be able to monitor the NAS itself for excessive IO or CPU using something like htop (even though that would be rather extraordinary if all that's going on with it at that time is a Borg backup).

On the topic of Kopia, you can go the easy route of setting up a SFTP remote pointing to your NAS. All you need is SSH access to the NAS (so your current tailscale/borg set-up qualifies as is) and a folder where the SSH user can write. You can then compare backup times vs. Borg (in my use case, going from 18-25min with Borg down to 3-5min with Kopia). If you then want to try the HTTP repository server, there is a container for that, but the set-up is a bit annoying (the password to decrypt the repo needs to be stored on the NAS, you need to generate a SSL certificate pair and have the client whitelist the sha256sum of it, etc). Personally I haven't found a substantial advantage to having the http repository server but YMMV.

I mean it's a commercial product - if stability issues were a common thing, that would be well known and published.

It's a mesh VPN so it generally has the same stability challenges as any other: Hamachi, Nord, Meshnet, Wireguard (which is the underlying protocol that Tailscale uses).

If the Tailscale client is running on the NAS itself, I'd consider that the first thing to troubleshoot.

NAS's generally don't have much extra processing power, and in my experience apps designed to run on them aren't the most robust. In this case, the VM/dock client has to consume all TS traffic, perform the encrypt/decrypt, put it back on the net (even if it's only with the virtual net) over to Borg, which then has to write to the NAS. The NAS hardware has to do all this, with whatever little bit of ram it came with, also while writing all these changes, managing stripes, etc.

While they advertise these things can run stuff like this, it just seems like an opportunity for performance issues when the processing power design was for a NAS/RAID.

I have an ancient NAS on my network that could probably run the TS client, I just don't bother. TS runs on all desktops and servers (for general access), but connectivity to non-TS devices goes through the server running as a TS router. That can be run on any device running TS - it's just a feature to enable and configure.

None of us are 100% self-hosted.