Finding the balance between budget and performance is key. At the €200–€300 price point, you are entering the "sweet spot" for high-refresh-rate 1440p (QHD) IPS monitors.
Regarding your question about 4K: I would not recommend targeting 4K at this budget or even as a "10-year investment" for a competitive player. Driving 4K at high refresh rates requires extremely expensive hardware (a top-tier GPU), and for games like CS2, the motion clarity of a high-refresh-rate 1440p display is far more beneficial than the pixel density of 4K. Monitors evolve quickly, so buying a high-end OLED today is a great 5-year investment, but "future-proofing" for 10 years usually results in spending more money upfront on tech that will be surpassed by mid-range models in a few years.
For your budget, a 1440p 180Hz–240Hz Fast-IPS monitor is the absolute "bang for the buck" leader. IPS panels give you great colors and viewing angles, and "Fast IPS" technology gets you close to OLED-like motion clarity for competitive gaming.
Here are the best options that fit your criteria: