Please avoid Samsung. It has issues (unless you flash LineageOS), like
https://github.com/koreader/koreader/issues/12099 and tends to yield ANRs much faster than any other brand, which may cause trouble if you use the program to run very long, blocking, tasks.
I wouldn't recommend Xiaomi or Huawei either. In general heavily modified android OSes tend to mess with UX and compatibility (They match the ranks in
https://dontkillmyapp.com)
From the PoV of support any *android* bug that cannot be reproduced on a emulator or LineageOS is considered a firmware issue.
Keep in mind KOReader uses the android framework in a different way than most android apps and relies on vendors shipping a non-broken, aosp-compatible, runtime.