I briefly tried the Max Lumi and the low contrast in combination with the low resolution (207 PPI) just made it impossible to use. The text was blurry and made my eyes water, trying to focus on it.
Later generations should have better contrast. Lumi 2 added Carta 1250 and Tab X further tries to improve that with a new thinner glass layer. The resolution is still low, though.
It's rumored that Onyx Boox will release a 13" Kaleido 3 tablet next year. And from what I've heard the PPI should be slightly better for 13" on Kaleido 3. On E-ink's own website they only mention 300 PPI for Kaleido 3, but I doubt it'll improve that much:
https://www.eink.com/brand/detail/Kaleido3. But if that holds true also for 13" like their webpage makes it sound - that'd be your answer.
If you need it now you might consider ordering both and returning one of them.