In this price category, the undisputed leader (in terms of frequency response characteristics and quality) is Beyerdynamic DT 990 Pro 250 ohm. I worked in them for over 15 years and still i have both 990 and 770. But 5 years ago I found headphones with a smoother frequency response and since then I have been working only in them. This is the Beyerdynamic DT 880 Pro 250 ohm. In 990/770, busting with low and high frequencies from which, with a subjective comparison when listening, they will sound better. But if you need to mix and do mastering, then you need headphones with the most flat frequency response. Whichever you choose, the headphones take a while to start sounding right. And you will need to listen to a lot of music in them to get used to the sound of the reference tracks. But … you can mix music well in any of the proposed ones.
It all depends on your skills and ability to hear. The choice of headphones is often well-grounded in controversy, but the best headphones are the ones you are used to and understand! To make friends with headphones, you need to spend a lot of time in them, listening to music, movies, commercials and everything. I wish you to find a new friend and helper! Here is some frequency response comparison and a link where you can do it yourself.
If you decide to buy a Beyerdynamic, I recommend taking replacement embouchures right away and changing them once every 3 months so that they wear out evenly. Otherwise, when replaced after complete wear, the sound will change slightly and you will have to get used to it again.