Despite some of the comments here, I suggest that you don’t overthink it; just buy an APC Back-UPS 600VA and be done with it. You have relatively low power requirements. The UPS will provide some surge protection (490J), several minutes of uptime, and a USB connection for automated shutdown.

The 600VA unit is less than $100 USD and replacement batteries are about half that. I’ve been using several of this same model for years without issue and we have many brown/blackouts being in a rural BC community. The batteries have lasted me 4-5 years.

You can always plan for something more significant down the road, if your hardware or needs change, but this should do fine in the interim.