I strongly dislike this system. It adds more things for the DM to memorize and makes character more similar instead of more customizable and interesting.
Following the example you used a lvl 20 rogue with legendary proficiency would have +23 modifier to Thievery while a lvl 20 paladin untrained on the skill would have +18. The rogue might be able to pick locks, while the paladin won't but when it comes to picking pockets the difference is minimal, since picking pockets does not require training.
I also dislike how adding level to pretty much everything means the values will be through the roof at higher levels. There is a reason why the proficiency system in 5e did not progress each level, it creates a situation where level is the only thing that really matters and everything else is just a minor bonus.
Really hope this changes in the near future because i really like Paizo sincerely want Pathfinder 2e to work for me, but what i'm seeing indicates things are going in another direction. One which i'm sure will not appease neither the fans of Pathfinder, nor the ones that are now playing other games.