Have you used these tools? Your main option is NUT, and it's not a great product. It crashes, it can't tolerate one of the UPSes being down, etc.
For my homelab, I ended up connecting my UPS via a USB cable to a Synology NAS and adding automation to shut down the rest of the servers to it.
I hadn’t realized this about NUT. I just started using it as part of TrueNAS’ built-in UPS monitoring support and haven’t had any issues thus far, but this gives me pause.
I buy that there is room for an alternative tool regardless of how crashy NUT is, but the technology choices for this are a huge turn-off IMO.
I'd just stick the server into a docker container, and firewall it from the public Internet. NodeJS is not something that I'd use for these kinds of tools, but it's not inherently bad.
Then use some rust file where you trust that the compiled file matches the source.
> Synology NAS
That's still NUT, just an older version with limited configurability through the DSM UI.