I suggest you check out Calibr2OPDS.
https://www.mobileread.com/forums/forumdisplay.php?f=238
I would recommend that you leave the choice of ebook reader software to the end users. You could perhaps make a few suggestions for different platforms. There are plenty of good free programs and apps. Calibre is a great program in many ways, but much too complex if only a reader is needed.
Even though I dislike the PDF format, perhaps that format would be better for your purposes? Your normal format could be EPUB, but to make things easier for the end users you could use PDF as well. It is quite easy to go from EPUB to PDF. But not from PDF to anything else... So make sure to keep the EPUB format as your main format.