I've no idea if this is doable but;
Now that brewing has the option of brewing in bulk up to ten at a time, what if there was the option given in brewing that allowed for rather than 10 stacking bottles, a large water skin or big bottle filled with 10 uses of what ever potion it was filled with. Charged items cannot be bugged by the storage system that I am aware of, and when the pc drinks the last 'charge' the skin/big bottle would disappear. So in effect, giving the brewer the option of making little vials of stacking potions, or one big bottle/skin that equals 10 if the resources to make ten are in the cauldron. A big bottle or skin could weigh the same as ten stacked potion bottles, right now that's 1 pound. Perhaps the NPC could sell a bigger bottle or skin that allows for filling.
If that's doable similar might be possible with alchemy? Alchemy allows for the extraction of essences in bulk amounts. Perhaps the npc could sell a larger bottle which can be filled with the equivalent of 10 essences which could then be made into an item with 10 charges of varnish application use and would weigh the same as 10 stacking varnishes, but can be stored.
As some one who plays a pc merchant, this would allow the compromise of still being able to offer potions and varnishes for sale. My character doesn't have a "hoard" of potions/varnishes so much as stock on hand for sale, and if she couldn't store those, it would be a choice made between mobility and continuing to offer those items for sale.