Did you try brucelee's
CoolReader3 fork? About a year ago maintenance stopped but it is still very good. It works on the newest models of PocketBook.
As an alternative, you can also install
KOReader. This is still actively being developed; there is a new stable version coming out every month. In the early days KOReader was rather unstable on PocketBook readers, but nowadays it works very well. And you have as many options to improve your reading experience as in CoolReader3.