I'm not an expert on inserts but I suggest looking at youtube, very useful videos there like this one:

https://pinside.com/pinball/forum/topic/vids-guide-to-ultimate-playfield-restoration#post-547755

There's some steps that don't apply in your case, but Vid's guide should give you what you need.

Im needing to fix cupped inserts and lower raised inserts.

I have fixed lightly cupped inserts with a self leveling clear coat, Everbrite,

I found a self leveling, evaporating coating less "risky" than over filling, or other concerns.
It takes twice the amount of "coats", but Ive been pleased with the results in a home use environment.

I cant imagine popping 20 inserts out of a playfield, unless it was a full tear down was in my plans.

For lowering, seeing the heat gun gives me hope.

I think Vids guides reflect the best, most professional way to do so, I guess Im a bit of a hack.

I should be doing it in a few weeks to share.