Solved.

RFXtrx433 appeared to be the E version, changed to different firmware (Pro 1, rev 1043). That fixed the temperature sensors.
KaKu units appeared to be ELRO which required special attention:
https://www.domoticz.com/forum/viewtopic.php?t=1194 Now also working.
Passed this level, going to ...

Dusted off an old 2015 Domotics install on Raspberry Pi. Originally running Domoticz 2.x and RFXtrx433 f/w 243.
Could not get my KaKu lightswitches or temp sensors to work. First upgraded RFX f/w to 1025. Then at least my temperature sensors showed up at the RFX. (These ones https://www.banggood.com ...