MBoffin

Just an update on this. I worked with Ultimaker Support and we went through a bunch of different troubleshooting on all sorts of things. While we worked through what might be creating the problem, they also helped me figure out things that would, in the meantime, at least let me print anything at all and have it adhere to the plate. However, we did eventually get it down to just replacing the flexible build plate. They sent me a new build plate and the difference was night and day.
 
Attached is a picture of the exact same model, printed hours apart, same printer, same spool of material. On the left is with all the tweaks and tricks needed to get it to adhere at all to the build plate (slower initial layer, lower build volume temp, brim, and so on), and on the right is on the replacement build plate with just straight up default 0.15mm Cura profile with no overrides on anything.
 

 
Some of the problems on the left are curled up corners, rough first layer, etc.
 
For now, I'm considering the issue solved. The only reason I say "for now" is that this is the quality that the printer was printing when I bought it a couple months ago, and it slowly got to the point where I needed to contact support. So I'm hoping it was just a one-off dud built plate and this one just stays fine from now on. I don't think it was caused by anything I was doing to the build plate, as I've been extremely extra-careful with it since I got it (only cleaning with 99% IPA, new clean microfiber cloths, rarely ever actually touching it with fingers, not scraping it, not actually touching it really with anything other than PLA, etc.). So, we'll see! But as I said, not it's resolved.
 
I will put in a good word for Ultimaker Support. They were super helpful, very responsive, actually understood what I was running into, and I never felt like I was just being walked through a support script.