Further discovery: I temporarily installed KDE Neon user edition on the same laptop and ssh worked fine. Reinstalling KaOS, the problem persists.
This seems to rule out any possibility that it's associated with the WiFi access point/router.
The fact that it works on KaOS using Ethernet makes me wonder if the problem is with the Broadcom B43 WiFi driver. Could that be significantly different between KaOS and Neon? I'll let you know if I find anything.