My advice is investing in better cable (+ eartips) and PEQ the AST first (Poweramp/android; Peace/windows). A low shelf for a little less bass (-2 db) and a high shelf post 7khz is key for accurate treble reproduction.I want to take a small step forward compared to AST, I bought them for $ 80 (a pretty good price for them), so I decide to choose moondrop kato or 7 Hz timeless, or maybe even pmv pp. I will focus on a fairly good separation, elevated sub-bass, non-nasal midrange and non-shouty upper midrange and high frequencies. Kato seems to be a good all-rounder, but perhaps he lacks a sub-bass. So if it close to kxxs, how it comparable to AST, except elevated bass? 7hz timeless and pmv pp seem similar and are suitable for my taste. The only drawback of 7 Hz is its mmcx, and they can have not good enough, for commuting, sound isolation and quite high sound leakage.
Kato is okay but still a single dynamic driver. Planars aren't commute friends, you may consider their terrible drivability and I doubt they would sound better than AST (except tonality). You can save a lot of money if you try to bring the best from AST