There is no 30% loss when you connect an external monitor to the graphics card. So it goes from the CPU via pcie 3.0 x4 to the docking station graphics card and on to the monitor. This means that this pcie lane is not used twice. Therefore, there is a 25 % performance boost. So you only lose 5-10% of the real power of a desktop graphics card.
In my opinion, such a station is worthwhile. On the road, you watch films, Office and a few programmes for work. At home, you connect the laptop with a cable and thus have a strong gaming performance and the laptop is also charged.

Unfortunately, the 30% power loss is true ... yes.
Whether it's worth it is always relative. For 3D modelling you also need a good CPU.
What kind of notebook do you have, and why does it have to be a notebook at all? Can it also be a whole PC ?
For the 600 you pay for the case, you can almost build a good basic system.