When monitors are new, they are the brightest they will ever be. As they age, they gradually get dimmer. It's not uncommon at all for a new monitor to need to be set well below 50% brightness to measure at 120 cd/m², which is the standard target value for LCD monitors when emitting a pure white signal. Since your monitor is rated at 300 cd/m², I'd be surprised if anything above 40% isn't brighter than 120 cd/m² with a pure white signal.

It's hard to tell how bright the screen is from the photo attached to your question because we don't know what exposure value was used. We need a measurement, such as x candelas per square meter (cd/m²). This can be measured with a screen calibration tool. We also don't know the difference between the signal being sent to the notebook computer's screen and the signal being sent to the monitor, because they are definitely not displaying the same signal. What does the monitor look like when the same thing displayed on the laptop is sent to it?

Is that just how it is in the sub 400$ range of screens, or are there much better options at about this price point?

One of my monitors is a 23" FHD (1920 x 1080) Dell P2314H. I paid less than $200 for it about 4-5 years ago. With it calibrated and profiled so that maximum output is 120 cd/m², the minimum output measured by an X-Rite colorimeter is 0.2 cd/m². That's virtually indistinguishable from when the monitor is turned off. Of course it is a smaller, lower resolution monitor than your 27" 4K Iiyama. A 27" 4K Dell U2718Q in the same class currently runs about $415 at amazon.