Thank you very much for your answers.
All my e-books are .kepub.epub and it's good to hear they'll work with no problems. The Likebook Mars is a good e-reader but its native software is a work in progress. I heard neoreader is more stable and overall superior.
E-Ink screens vary between units. On paper, they might have the same screen technology and PPI but you notice differences close-up. Some screen backgrounds are lighter than others. Text contrast and clarity vary between screens too. For example, I owned the Kobo Aura One and its text appeared darker in comparison to the Aura H20 (it is not related to the slight PPI difference). The Likebook Mars's screen is also not as good as the Aura One.