For his career, Hiura is a much better hitter against RHP than LHP. Even with his bad 2020-2021 numbers, his K rate against RHP is roughly 33%, which is manageable when his OPS against righties is mid-800s. Against LHP, his career K rate jumps to 37% and his OPS plummets to sub-600. Ideally he gets his K rate in the 25-30% range, but that can be said of a ton of today's hitters in MLB.
Keston's 2019 saw 18 of his 19 HR in the majors come against RHP, an astounding reverse split. Even that year at times he struggled with hitting the fastball - the all-fields aspect of Hiura's hitting seems to correlate to taking offspeed from righties that is breaking away from him to right center with authority. For whatever reason, he doesn't seem to be able to handle offspeed from lefties breaking towards him.
Getting Hiura consistent MLB ABs would be much easier if he could hit left-handed pitching - he's still young enough to develop that, but it's on him to do so if he wants a long MLB career. I think he's going to get an extended run in Milwaukee at some point this year, and how he performs during that stretch will determine if they view him as their DH for 2023 or if Hiura's traded in the offseason.