That is why I was asking if someone tried Cool Reader 3 made by jotas.
I am using it on my still favorite android reader Sony PRS-T2.
It is an e-ink specific reader with enhanced support for folder-based libraries while almost all the other readers are made for android phones: the official Cool Reader 3, AlReader, etc.
Other readers usually require e-ink unfriendly scrolling. But CR (jotas) handles everything with next/prev buttons: in the built-in file manager, in the (enhanced!) recently opened books list, in all the tabs in settings. No stupid animation, no unnecessary bells and whistles, things arranged on the screen so that the most frequently needed ones are immediately available, etc.
It also has specific support for landscape orientation (I prefer it on 6" readers because I like wide margins): some stuff is OK in the official CR in portrait orientation but requires scrolling in landscape. No such thing with jotas - taken care of
It also has several of its own e-ink refresh modes to choose from.
I prefer having books arranged in nested folders instead of some virtual collections that tend to choke with a bigger number of books.
Not only the indexing may take hours, but also the repeated re-creation on the fly of lists in collections may be unacceptably slow as well.
Because Cool Reader by jotas specifically supports C67 there is a good chance that it supports C67ML Carta 2 and T62 / T63 as well. At least their possibly interchangeable firmware with C67 family is a good sign.