"We do not limit the amount of varnishes one can produce, but it is on purpose that we keep their production slow and one at a time. Bulk crafting would also drastically increase the speed one can master a craft, which isn't something we particularly want."
From my perspective, this has little to do with slowing down mastery of alchemy, sounds wrong, and just makes for a very boring/unfun game experience.
Things could have very well changed, so I could be mistaken, but I recall the DCs for dissolving things into essence the be the more difficult DC, and where the exp comes from, once you get to say, level 25 in the craft. I'm wanting to say the DC for turning most things into varnish around the middle/higher levels was lower than the to make the essence. Example, greater hag heart has a DC 35, the essence they yield would have a 25, or 30 DC, to refine into varnish, and net zero crafting exp at that point. So in cases where the raw item yields 3 essences, the DCs for resin yield zero crafting exp, after a point, but it takes 3 times longer to refine into varnish, depending on the number of raw things you start with.
The true slow down from mastering, at least for alchemy, is the scarcity of resources after you hit level 30, and the amount of exp you gain from a successful craft. I think for a brain, I get around 30ish per success. It's been a hot minute, so my memory isn't perfect on it.
e: reworded some things so they would make sense.