Personally, I feel like you can have a more consistent experience and results with good headphones over monitors. A good pair of monitors, even used, will be $300 maybe? I haven’t looked in a while. A great pair of headphones to mix can be had new for $100.
With headphones you can pretty much disregard room acoustics, which can have drastic effects with monitors. You also don’t have to worry about things like keeping your head in the sweet spot of the monitors, or working late at night and making too much noise.
My biggest sell for headphones is what I’ll call “transient training”. Being able to pick up on what compressors are doing is doable on headphones and much more difficult on monitors for me. If you find a pair of “fast” headphones, small changes are much easier to discern. Planar magnetic headphones are amazing for that, and the ones I use are the ones I mentioned earlier that can be had $100 new.
At this point, I only use my monitors as another reference, or when playing / recording, but not for “critical” listening.
Just my perspective, but I know of a few high-end engineers that do the same.