Well the thing is:
AFTER the discard of all solids you should always have your solution filtered.
This means: After you combined all the liquids from your cook, definetly let it flow through an 1/2 cotton t-shirt to get rid of little particles. They destroy the separation process with Naphtha if they are too much. At this stage I recommend filtering the BIG particles (what you have when you work with shredded and not powdered bark - which I recommend) with a sieve and THEN use that t-shirt as it will not clogg so fast.
I guess in any way you may have also filtered your original liquids.
But then when concentrating, more stuff precipitates. This is again just red sludgy stuff which can be discarded, but when you concentrate it and then more junk starts to crash out you need to let it flow again through a cotton T-Shirt (or something similar).
But keep attention to squeeze the junk inside that T-Shirt too to get most of the liquid out of it. Every liquid stuff contains Goodies so you want to separate the junk as good as possible.
And THEN if you filtered your stuff well and there are no precipitates / sludgy stuff / whatever, then your soup will be 100 % liquid with no particles. So this means as long as you always filter the solution from sludgy junk, you solution will be definetly be stirrable by your stirrer.
A high concentration of dissolved molecules in your soup will NOT make it viscous / not-stirrable. Only if (too much) solid particles are left in the soup, then it becomes visous.
Also having a lot of particles in your soup will highly increase the time for the layer Separation of Naphtha + Soup ...
1k bark to 500 ml is quite much, so can't say this from experience, but any reason why you could NOT stirr it would be that you have too much particles in it and those can be filtered off, as said.
But keep in mind that there must be a special concentration when even Spice crashs out ...
0,5 kg Bark to 500 ml is no problem I think, 1 kg should be fine also. But just keep this in mind. :thumb_up: