For a new guy here, your post is rather well done, comprehensively described. Considering all the things you've done to narrow the problem, I'm surprised it's as bad as it is. Only one thing jumps out at me and that's the relatively low temperature for the nozzle, although it's usually too-high temperatures that can generate excessive stringing. I'm trying to envision how a too-low temp would cause the problem and my alleged mind comes up empty.

I found a useful web site but it appears that you've already found it and/or covered everything listed and then some.

One thing you've not mentioned is a change of filament. If your spool has been out in the environment and has collected a bit of humidity, you may see a benefit by dehydrating/drying it or using a new previously unopened spool. It's grasping at straws at this point, but should be a consideration.