Bacon Bits
Hero
Also, isn't Bounded Accuracy specifically about attack rolls and AC?
Expertise is about skill rolls.
No, bounded accuracy is about making every die roll meaningful. It's about never making it so that PC #1 can't possibly fail while simultaneously PC #2 can't possibly succeed. It doesn't matter what kind of die roll it is.
The entire point is to stop making rules that tell people to roll dice when the die rolls don't matter. The DM should only call for a roll to challenge the PCs. Certain success is not a challenge. Certain failure is not a challenge. If the task is so easy or so dire, just narrate it. Dice are what you use when you need equitable uncertainty of outcomes.
That's why very high bonuses are bad. That's why the core game essentially limits you to -1 to +11 without magic, and tells you that DC 15 is moderate, DC 10 is easy and DC 20 is hard. If someone has a +17 bonus, then the DM can't set a DC that meaningfully challenges both that PC and the non-proficient PC who still has a -1.
Personally, I would rather Expertise give you a +2 or +3, or otherwise worked like a luck point.