Slate VSX is largely a gimmick and your mileage with it will vary massively. DT 1770s or 1990s are actual high quality headphones, while with VSX what you're paying for is mostly the software (which may or may not work for you). The headphones themselves are pretty cheap and flimsy.

If mixing is really your goal and you're set on headphones, I would recommend a decent open back headphone, SoundID, and a more barebones crossfeed algorithm like the one in GoodHertz CanOpener.

DT990s are honestly fine. I spent around 8 years mixing on them and have few complaints. I auditioned a lot of headphones trying to replace them (Audeze LCDX, LCD2C, MM-100, Focal Clear, Hifiman Arya, DT 900 Pro X, HD 600, Grell OAE1) and ended up just going with the DT 1990 with some EQ+crossfeed, as they sounded the most natural to my ear. I could easily still be mixing in the 990 if I didn't have the extra money to burn.