To work on a daily basis I use AKG K-240 MKII headphones: they are very precise, very pleasant headphones, they do not cause any fatigue because they are semi-open, (announced bandwidth: 15-25000Hz). I kept and still use from time to time the excellent version (made in Austria) 600 Ohms K-240 Monitor.
For mixing: Focal Alpha 65, but I check with small PreSonus Eris E3.5 BT speakers, and Zadig 301 hifi speakers from Cabasse. There is no universal and perfect listening solution, the only rule is to know the qualities and defects of your own system. And check the result on different supports.
Concerning the extreme frequencies, I usually cut (with an Eq in Logic) below 30Kz with a curve of 24db, and above 17000Kz also with a curve of 24db. There is no point in going beyond this, the vast majority of listeners and in most situations cannot perceive extreme frequencies. It’s good to manage space, and don't disturb what will be most useful to the hearing. As often said, the art of mixing is to make room and avoid everyone stepping on each other’s toes :)
'On ne devrait jamais quitter Montauban' (Fernand Naudin).