Gonna throw a wrench in this whole batch sparging reduces pH concerns. I get the science, less time for the reaction to occur, but it depends on your water. With my 90ppm alkalinity water, batch sparging is no different than fly in terms of pH. I have the data to back it up. Also, acidifying sparge does more than eliminate tannin extraction, it drives the overall wort pH down and once you start comparing your final pH to your favorite commercial beers, you will realize you need more acid in your process than you once thought. OP did himself a huge favor looking at this already.
If it is astringency it is from polyphenols, polyclar will fix that in a snap. Like I said tho, astringency I think is more a puckering/drying the tongue feel and back of the throat is classic high coho hop (some guys love this). Most likely it was a combo of all, long sparge, iffy RO water, and the CTZ.