It is often surprising how many such debris fragments are loosened on old Vinyl by an ultrasonic clean!
Another technique that can be used to loosen embedded fragments that even ultrasound won't loosen on its own is to treat the vinul with ArmorAll... (yes the automotive vinyl treatment) - given time (several days to a week) - a very very light treatment with ArmorAll will gradually migrate between the grit and the vinyl in which it was embedded, loosening it ... a subsequent ultrasonic clean, will then release those additional "boulders" (from your needles perspective)....
This can occasionally do magic with older, mistreated records.... sometimes those "pops" aren't damaged vinyl so much as embedded debris, pushed down by a multitude of dirty plays....
Once released, it still leaves behind vinyl damage, but it is far far less audible than the "pop" caused by the embedded chunk.
And yeah, in a perfect world, you wouldn't play mistreated vinyl, but some rarities are just that.... rarities... and if you can't find a way to play them, then they are lost for good.