You can just render to the 1bpp text/4bpp images monochrome 300dpi target; perhaps some tone mapping due to the limited contrast ratio of the eink vs. your OLED should be done at rasterization time.

Then just map the resulting 16 brightness levels according to the contrast ratios/progression they have on your eink. Use phone camera and adjust whole image contrast/brightness/black-intensity sliders holding them side by side until the grayscale matches.

Then just pick pixel RGBs from a gray-bars test image you have opened on the remarkable before you took the picture with your pixel 7 pro. Might not easily be done on the phone but can let a python using AI grab those for you. (If it's wrong it'll be obvious or so harmless it doesn't matter anyways.)

Then you take that rasterized-to-1/4-bpp-djvu apply the color palette you just built yourself, and zoom to 300dpi.

Other than the screen backlight which you've taken care of when you held both devices side-by-side and played with the sliders, there's no substantial difference left. Should be pretty representative. Don't look with too much of a magnifying glass though the phone doesn't have exactly the same dpi so there are resampling artifacts your eyes resolve if you're too close to it.