@Zelme you got the best answer immediately under your post.
Make Expertise bumps 5 per bump, and the system will work just fine. As indicated, it would only take 20 bumps per slot rather than 30-35 which I believe is the current number/target.
The best solutions are often the most simple. No need to reinvent the wheel here.
People have an issue with “time-gated” content, which is why the daily thing is a turn off to some, I think people don’t quite get there are in game activities that bump expertise with NO time-gate, like dungeons.
Right now it only feels bad because a player hits 60, runs a dungeon or two, gets some gypsum casts, and they look across the board and feels they have barely moved their expertise.
Just make it 5 per bump, and players will be able to feel they can more easily, smoothly go from 500 to 600.
If you want to further “bridge” this gap, you might suggest enabling people to use Umbral Shards at 590 expertise, rather than having to be 600 expertise in a slot.
This would GREATLY help, and would be intuitive, considering we can use Umbral Shards on items at 590 already.
So now what you have done is moved the Expertise System Grind from 500->600 down to 500->590 and at the same time gone from it taking an average of 30-35 bumps to hit 600… Down to 18 bumps to hit 590, and then can use Umbral to “speed up” their expertise above that.
This will VERY LIKELY solve this entire issue.