The a/b partitioning scheme doesn't eat into internal storage—on a Google pixel 1, the a/b scheme takes only 320MiB more than the a-only scheme. Source:Android Developers https://source.android.com/devices/tech/ota/ab/ab_faqs
I realise this isn't maybe the most helpful of answers, but I thought I'd post it anyway in case someone else finds it useful.
The phone was under warranty, so took it to the local Samsung store. The support assistants commented that they'd "never seen this error before". It went in for repair and emerged fully working again.
When I asked what the problem ...