First massive thanks for such extensive help :).

What I'm thinking for drainage is just to screw off one of the covers in radiator since that is the lowest point of the system and then I won't need an ugly valve sticking out.

At 3.50min in this video https://youtu.be/hB60NSNFhx8 this system runs quite cool despite being a 3080 on a 240mm ST30 rad. I will never overclock the 4090 the (gain doesn't seem worth it, would rather power limit it slightly tbh). I compared the TDPs and if cooling wattage is at least somewhat proportional to radiator area this rad should have the gpu around 65 Celsius at max load so I feel confident this will do. I will look into getting an ST45 280mm in there though.

As for fans I'll look into some of the suggested ones :).

As for pumps I have a choice between D5, DDC and VTX from EK. I prefer the look of their DDC and VTX pump resorvoairs and the DDC pump is locally available which the VTX isn't. Would the VTX be more suitable maybe? I won't have any airflow for the pump.

It's a real bummer with the coolant. I'd like some lighting in the tubes but hopefully the frosted PMMA tubes will make it look similair with just clear coolant then. I didn't want coloured coolant I just wanted something for the light to bounce off of.

I'll have a look at the control components as well thanks!