Decided to have a go with that Yolink stuff Timmmay suggested. Here's hoping (it never sends anything except a test message...)

aqara water leak sensor. Cheap, small, works with zigbee and ConBee II directly under Home Assistant

I simply connected a rain sensor surface to the water detector pins of a second device. Now it also immediately reports the onset of downpours so that the laundry can be saved outside (if you are quick). 😆