Thanks for the info, Martin.
I was thinking about getting two heaters with 300W each. I still need to make a reservoir where I can submerge them, though. I already use multiple air in sensors and have a rig where their distance to the radiator is always fixed.

I probably won't be able to get results as precise as the ones we got used to from your testing, but an improvement over the setup I have now should be easily achievable.

Changing fan rpm is something I have also noticed but don't have a good way of controlling yet.