Update: three hours after the restoration attempt: success. Mac restored & working fine. Time Machine failed me for the first time since 2007. Restorations from both a system snapshot & a 4-day-old Time Machine backup failed to have any beneficial effect.
But I make clones, too, using both Carbon Copy Cloner & SuperDuper! apps. I chose to use CCC for the restoration, its clone was on an external SSD.
Booted from that, made sure all apps were updated & restored it. Took under 15 minutes, transferred 6.6 GB data.
Mac is fine now.