Additionally, I believe that there exists an additional method to enable greyscale in Developer Options if you have that enabled.
I think that I suffered more eyestrain than usual, mostly because I could not tell icons and other UI elements apart. Yet I don't have this issue on the E-Ink device, interestingly, even though I use many of the same apps e.g. Telegram.
I would say that the difference is due to the reflective vs radiative screen, not the greyscale palette.
So for me, the whole point is I won't be using it enough to notice eye strain or something like that. If I'm messaging or doing something the device is designed for, it's a benefit for me to be able to have it in full fidelity.
For greyscale you use shortcuts. There’s an automation tab, you set up on a focus mode being enabled then “set color filters on” and then the opposite for that focus mode being disabled.
I have a deep work mode that sets a blank Home Screen, turns off always on display, sets the phone to greyscale and only allows notifications from my wife.
It feels like something like this gets you most of the way there and you can then have different focus levels that ramp up potential distractions. All without carrying two devices or giving up the modcons.