Amusingly (or sadly) enough, with 110ms RTT, SSH is slower than Tailscale. See my adventure at Is TrueNAS is shipping an SSH binary that throttles connections over high-latency links?
Time for plan C: zrepl. No VMs or LXCs or Docker containers. This should be doable with a shell script and a cron job. I cannot believe I am forced into doing this due to complete lack of tools on TrueNAS (you had one job, TrueNAS!), but so be it.