I do not understand how it can be used early game. Pump is mentioned as a reason but the pump will try to fill the tank connected to a relief valve with all fluid, and not just excess that should be wasted. It feels like there is a zero purpose to it as the relief valve could be connected directly to the system via normal pipes to achieve the same result.

Maybe if I had another tank infront of the pump and controlled the pump via circuits. But then I still would not need the relief valve and could just use the pump controlled by circuit network.

Unless the only way is separating production system which would pump into a tank, this tank would connect to a consumption system and also have a relief valve. I find it quite weird and use circuit network instead.

You can accomplish almost any basic need with just valves, not circuits. Valves are meant to be used with tanks, not pipes. Tanks should be filled using pumps. Your inputs to the tank should be using 1 pipe and your outputs using another. If you're trying to do everything with a single pipe it's not going to work. You need pumps to keep acceptable flow rates over any sort of distance, and that means one way pipes. People can get away with some bad practices in vanilla since there are so few fluids, but a lot of this is pretty basic fluid management across any mod that makes significant use of fluids.

The main role of a topup valve is to fill a tank from a source that intentionally produces a product which can also be produced as a byproduct. The byproduct source fills the tank with just a pump and no topup valve. The main product source fills it with a pump into a topup valve into a tank. That way the byproduct source is used preferentially, and you only spend resources producing that fluid if the byproduct isn't enough. The relief valve has a higher threshold than the topup valve, so that way you're never manufacturing a fluid just to vent it, but you will vent excess byproduct.

The role of the auxiliary valve is when you have more than one type of output and want to prioritize one of them over the other. It can also be used as a makeshift relief valve for people trying to do what you're trying to do, attach a relief valve directly to a low pressure pipe. But that's not the intended purpose in a mid to late game system requiring higher fluid throughput than you can achieve with a two-way pump-less system.

The valves are a nice idea, however with the relief valve specifically I find it a little bit annoying that the threshold is larger than the early game pipes. That means both the valve and the pump have to be directly connected to the tank, because any pipe segment in between will drop the pressure too much. The combination is quite rigid and unwieldy, which makes me prefer circuits for this task. I wonder if there's a way to allow at least a single pipe segment between the components for increased flexibility without promoting bad practices?