I’m going to reply to my own question, in case someone finds it useful.
It turns out this had nothing to do with an interrupted dom0 update. The problem is that when I first installed qubesos, many years ago, the auto-partitioning tool in the installation process created very small boot and efi partitions. These were just about adequate for the smaller kernel images of back then. However, as image sizes got larger, I eventually ran out of space in the efi partition.
There is currently no way to resize an efi partition with gparted. There are hacks that involve deleting the partition, shifting-resizing everything else in your disk, and then re-creating a whole new efi partition, and finally copying the old content into it (but this is pretty cumbersome and likely to result in data loss anyway).
To cut a long story short, I did a system backup, deleted all existing partitions in my disk, and did a fresh qubesos install (which is just as well, because the system was getting sluggish and bloated after years of in-place upgrades). If you find yourself in the same situation, make sure you select the unintuitively-titled box “claim additional space” in the installation menu (this will allow you to fully remove old partitions).