(This log looks like a wall of text so I'm going to see if I can underline-highlight the important parts.)

Why 2160px again?

I have to remember that the main requirement is that I need enough pixels to work with. While I wouldn't be able to multitask all that well on a single FHD monitor, a 2160x2160px display can fit dual 1080p or 2:1 aspect ratio displays without even having to turn my head.

I'm still going to aim for 90PPD at 90Hz, as those two metrics are the most important for comfortable work over multiple hours/days. I'd also like to add "that doesn't cost £900" onto that list, because that's near the expected BOM cost if I were to use the 2560px screens, assuming that they cost around £650 after import taxes.

This does mean that the FOV shrinks down to 24 degrees, or 25.4 degrees at 85PPD. This is the same as an iPad Pro 12.9" that is 62cm away from the user's eyes, so it's not small in respect to other portable display devices. 

Screen brightness concerns using the 2160px display can be addressed after solving chromatic aberrations. Spherical aberration seems fine(the screen would either look more like a CRT or a spherically curved monitor).  

Curved or flat optical combiner?

I've been considering using the curved optical combiner, but that likely would only work for the 1" displays if I want to keep the FOV at around 30. It also seems like I'd still need lenses anyway, which is what I was trying to avoid with the curved combiner idea. 

The flat, circular combiner seems like the best solution still, since it is the most likely to achieve 90PPD, which is unlikely to be true for other technologies currently known. They are made for different requirements, whether that be augmentation of visual objects, giving instructions or showing small snippets of useful information, not for use with 3D CAD and programming toolchains like with SimulaVR.

Custom PCB and MST

Like I said in the previous log, I also have the opportunity to create a custom PCB that is hopefully cheaper than the £80 (excl vat) ones on AliExpress. 

I'd still have to use the MST hub I found a whilie ago, because it doesn't seem like I can access any DisplayPort 1.4 chips (or the datasheets to them so that I know how to connect and program them). The TC358860XBG is available (since 2017) and has a maximum throughput of 600 million pixels per second. 

(2160^2 * 90) / (600 * 10^6) = 0.69984
                            = 70% utilisation

For systems that don't support MST, each eye should see the same image and should still be perfectly usable. For those that do, it would be possible to have a DP_OUT port (or USBC ideally) so that a user could connect a portable monitor to the headset to show other people their display, for work that requires accurate colour reproduction or to have one main monitor and infinite ghost / virtual monitors around it.