The other suggestions are evenly split between major hardware changes (even though I've pointed out that we don't want to do anything to the hardware until winter shutdown) and making the file system read-only, which will work but will make it hard to update the software or the images. One idea that works for the images is to put them on the network (eg the NAS Box). This will do nothing to allow the OS to be updated.It is pointless anyway to measure the 5V supply -- by the time that collapses, the battery is absolutely dead, and it's too late for a safe shutdown. This is because the DC-DC converter will maintain 5V out as the battery voltage falls, as the expense of drawing even more current from the battery, discharging it even faster.
(If we'd used a Pi 4 (at nearly five times the price and 2-3 times the size), we could have used a network boot.)