I certainly don't claim to be an expert, just a guy who tries things to find some things that will work and a lot that don't. I think you'll find that the fiber based papers will lose their slight curvature within a short period of time. My prints usually flatten on their own in a day or two at the most. I use 20"x30" foamcore board with little strips glued to the top and bottom to lift the next board up the thickness of the foamcore and to keep each one in position side to side. That leaves enough air gap at the ends so that the prints can breath for as long as I leave them in the stack without picking up a lot of dust. Always put an empty drying board on top of the stack. I find that with the 17"x25" Fiber Based papers, I can put the next foamcore board on top almost immediately without the print touching it. After you take the prints out, you can stack them in a flat place, a used print box, or whatever with good acid-free interleaving paper, such as Westminster paper, between prints. They will be totally flat for mounting in short order. I don't think it's a good idea to frame them behind glass or plexiglass before at least a week or two have passed, as you might still get offgassing and have to take them out of the frames again to clean the inside of the glazing.
If your machine is new, the pizza wheels will eventually polish down so that they won't leave marks. At least that's been my experience. Try running a sheet of 17" wide fairly stiff stock through the printer upside down repeatedly. Don't print on it, just advance it through multiple times. It didn't take very long for me to totally eliminate the little fine pizza wheel marks by doing that. Maybe a half dozen passes once or twice. The back of the paper should be calendared smooth, and not something that will leave fibers behind. A thick glossy paper like your GFS run upside down should do the trick. Yes, you should use the sheet feed, not the rear feed on large paper. See my post above for how to use it for best results.
Settings of Wide (not Wider) and 5 work well for me. Even Auto seems to work OK.
For selecting papers, I subject them all, whether glossy, matte, luster, or whatever, to the same initial test. I print the calibration file from Quadtone Rip and read the patches on the black, gray, and light gray ramps and then plot the densities as curves in a single Excel graph. All that calibration file does is push ink in 20 steps from minimum to maximum. No possibility of screwing up a setting. It just pushes ink. That test tells me very quickly how well the paper takes ink. A lot of highly touted papers block up in a hurry and then lose density! Some produce bumpy curves while others take ink in very smooth curves and get decent dMax numbers. Several matte papers can't get a decent black to save their souls. If you get clean, smooth curves and a good dMax, you can be assured of a relatively easy time of linearizing or profiling for the paper. Otherwise, all bets are off. Subjectively, too, you can compare their colors, surfaces, reflective qualities. You'll immediately spot bronzing and gloss differential problems, and you can tell which papers are loaded with brightners (even if they say they aren't), and which ones are not (not that I think brighteners are all that bad, but that's another story). For matte papers that I've tested so far, I find one to be outstanding in many respects, and that is Epson's Velvet Fine Art, though I wish they'd go ahead and put out a version with some brightener, or a better bleach job, but it's a very fine paper with few faults. The dMax is the best of the matte papers I've tested. I haven't tried Hahnemuhle's William Turner, but some say it's hard to match the depth that you can get from that paper. For a very white paper, Red River 60# Polar Matte has a fairly decent black, excellent curves, and they even make a nice dual sided version. If you compare it with VFA, though, the blacks will look weak. Just don't blast it with light that's too strong, and it will be very impressive. Yes, it has OBA's. You have a right to worry about them if you wish.
Good luck and Aloha,