sl;
"stationary at tickover, the gauge quickly rises to the top of the green. When I rev the engine (still with the car stationary), the needle quickly falls back to normal. I don't even know, is this normal? "...this is actually quite typical, and nothing to be concerned about, due to a combination of low airflow through Radiator, and slow circulation by the Water Pump, so don't suspect it just yet.
If your Cooling System is free of air and Temp Gauge is working normally so that you can trust it, you can just monitor it, and take note of any changes in the action.
Clan already touched upon the Instrument VStab as a possible reason for hot indication, and this seems not to be an issue here, but it bears repeating to make you fully aware of this important component...your car has one of these supplying several instruments, and you might/should be interested in reading about it and the indication errors it can cause. See:
https://www.sw-em.com/voltage_stabilizer.htm
and
https://www.volvoforums.org.uk/showthread.php?t=284381 ...and read about Graham W's experience!
Cheers