il faut bien expérimenter

You can simply use a "water temp" - "ambient temp" sensor (with a low pass filter if it varies too much)

For the fan curve to avoid having your fan speed jump, just set a curve like that for example, and tweak the base, and maximum speed depending on the temperatures you want for the water :

And since you have a flow meter, you could have some fun and experiment with your pump speed depending on how your loop is set up.
For this to work you would need a temperature sensor before, and after the rads, and have all the rads in series (no waterblock in between rads).
You can then measure the water temp difference between input and output of the radiators (in this example i made a virtual sensor of it).
With water flow value, and knowing the specific heat of water, you can create a virtual sensor to calculate the heat really dissipated by the radiators, in watts.
You can then play with pump speed and see what it changes in terms of dissipation.All you'll see is spikes when the flow increase suddenly... then as water in and out temps stabilize after like 10 seconds... the dissipation is the same, no matter what flow.
.. and it's a nice sensor to have in the dashboard :p
In the end, for the pump speed, you'd just want to set it to 100% and look at your GPU temp under load.
as you lower the pump RPM, the GPU temp will climb extremely slowly.. like maybe 1 or 2° when you're down to 50% speed... keep going down and there's a moment you will see it increases a lot faster. In my case for example, if i go below 40% the temperature shoots up by 10° quickly. so that's my lowest speed.
From there, either keep the pump speed fixed, or set a curve that doesn't go below that threshold.
That's just how i set things up, you'll have to experiment a bit to find good values that keep things cool and/or silent as you like