For almost all metrics, the 2560px microdisplay is better than the 2160px IPS, but the thing is that burn-in is a possibility and those SiOLEDs are expensive. Not even just a tad, but an order of magnitue more than the 2160px's. The gap reduces slightly considering I'd likely have to upgrade the backlight. Project Northstar seems to be fine with the brightness of the 1440px edition of these screens though, so it potentially could still be usable without the upgrade. I'll have to see if anyone in that project has used the headset outside though.
Additionally, I can't get full resolution @ 90Hz, but I can get 1920px upscaled @ 90Hz. 75Hz is probably fine enough, but it just adds to the difficulty of justifying the price.
The 2160px also is a low enough resolution that I could create a PCB using 2 Toshiba eDP chips to process the data. I might also look into the chip used for DP MST, but using an AliExpress hub for £30 is a bit more likely.
Anyway, it seems that off-the-shelf optics aren't going to be good enough for what I'm trying to do, and would have to spec #SecSavr Suspense [gd0105] to be able to manufacture finished optical elements, and continue this project after success. Additionally, those custom PCBs could get expensive since I'd very likely need more than 2 layers for the board. That's also something else the Suspense needs to do.
Lastly, that acrylic visor is still kind of expensive. I'm not sure if 3D printing in some future #SecSavr Syrum [gd0141] could be cheaper, but at least I'd have more geometric options for it.
Essentially, I've come back to the same conclusion I started the project with: A TyMist solution likey doesn't exist if the Suspense doesn't exist.