All projects describing any sort of NAS setup usually stress the importance of feeding it through an UPS, even if it's not on 24/7.
Many of these homemade UPSes (especially the Pi-based) use repurposed USB power banks.
I found the idea interesting, except that my disks need 12V.
Then I realized that I had this Suaoki battery booster in the garage. This little lifesaver is quite impressive for its size and weight : it will start a car or motorbike with a totally flat battery, just like that. Also you can ignore it for weeks, and when you need it, its internal battery is still full.
It is charged through a 15V wall wart, has two 5V USB outputs and delivers 12V through an EC5 connector.
So of course I had to try as an UPS, by replacing the 12V PSU brick and its barrel connector by the booster with an EC5.
As you can see, the booster is feeding the disks and topping off its own battery via the 15V input (only the fourth LED is blinking).
I haven't used the USB ports yet, but they might be able to power a Pi4 with a double connector, will check that.
Okay, y'all can relax now, this should be the last update -- barring an unexpected SATA solution for the Pi coming out of nowhere, obviously :-)