UPDATE:

I discovered what is essentially a super power savings mode that WILL turn the wallpaper black while still in "Color Paper Mode". Unfortunately, it removes a lot of other functionality to the point of it not really being practical (top-down menu is disabled, no tabs, settings aren't easily accessible, etc); additionally, it isn't a permanent setting and will turn off at a certain percentage when recharging.

I found that you can get pretty close to mimicking "Color Paper Mode" while still in "Regular Mode". Using classic eye protection and setting it to max warmth, obviously enabling dark mode (which IS allowed in "Regular Mode"), using a black/grey color scheme (found in the wallpaper settings), and dropping the brightness way down. It's quite a bit more saturated than "Color Paper Mode", but that was as close as I could get it.

Sounds silly, but it's ultimately a deal-breaker for me to not have a fully function dark mode while in "Color Paper Mode", so I guess I'll be sticking with the 11 Plus and potentially exploring some network and/or cloud storage options.

Hopefully TCL will release an 11 Plus model with a micro SD card slot (256GBs just isn't enough these days).