I would had loved my latest gen ipad to be as squeak-proof as my cheap 9.7" ipad. Lets hope Apple improves on that aspect soon and make us all happy without having to resort to ...skin and snake oil solutions.
I can't call it a defect, since for most people it's not a problem, but definitely a manufacturing process "insensitivity". It's very frustrating buying an expensive product that was meant to make life easier, in the process realise that it has some deal-breaking traits that the company doesn't seem to care about fixing.
I also understand how altering the dynamics, texture or the appearance of the painting medium in any way (in this case, by adding a screen-protector) might be killing all the pleasure or productivity while working. I recently tried some dry chalky pastels and the powdery feel made me want to rip my arms off. Sadly some things don't work for us.
For my use case I find the diffusion of the screen-protector "charming" and not the least prohibiting. Here's a a recording of my ipad where I am comparing a part of the display that has the protector on, and a circular area that doesn't. I am zooming out a grid of black and white dots of 1px diameter each, it can't get finer than that. The circular cutout diameter is 1cm (0.4") so this is a real close up video here - you can even see the individual pixels of the retina display. I'm more frustrated by procreate's aliasing/moire-inducing zooming technology than by the diffusion caused by the protector to be honest.
I've also stopped cleaning my screen protector! I used to wipe my old ipad daily and even clean it with dish-soap once in a while, and the least fingerprint would show. Now that I don't clean it EVER (only wipe it a little once a month to get dust particles and dried saliva droplets off) fingerprints are invisible. The display apparently is evenly coated in grease. Think of it as a frying pan - From what I hear people in the US go to considerable lengths to protect the grease buildup on it - iPad are designed in California so ...perhaps it explains :)
Regarding friction/resistance/gliding: Again, I know nothing of your art habits and work style so I'm always speaking judging by myself, knowing that nothing that I say might work for you.
In the past I was drawing on my old ipad using a smudge protection glove (3 finger glove) which would allow me to glide my wrist freely on the display without friction from sweat and the large contact surface. Come to think about it, before the ipad, while I was using wacoms, I was used to drawing without resting my wrist on the display, and only using my pinky to pivot, but the touch-screen would be confused and assume that I was doing two finger gestures! I had to relearn and use the wrist, and then had to deal with friction. Hence the glove.
So in your case, I wonder if saturating the display with handoil™ (we might have found a successful product for a niche market here!) and then using a glove, might compensate for the loss of ease-of-gliding..!? dunno!
I hope you find your solution Lizzieat, because if I had to keep up with the squeaking a day longer, I wouldn't be using the ipad for art. It gave me shivers like people scratching their nails on the blackboard.