[stumped]

i used to read sideloaded epub books on a Kobo Arc tablet, which had the kobo reader app built in. That was my first ever tablet.
i just copied the .epub files to the tablet then let the reader app import them into wherever it wanted to hide them. I suspect they go into some hidden.db file but I never really cared. I think I may have tracked it down one time in order to delete it when it got corrupted, actually, & I think it was something with a .db extension, but |I may be confusing the Arc tablet with the Aura HD e-reader that I bought later.

it has been a while since I had that tablet but I think i used calibre companion or some other similar app to get the books to the device wirelessly.
also, for a while after buying another android tablet , I continued to use a sideloaded version of the kobo reader app as I quite liked the interface ( until Kobo " improved" it with changes to how it did line spacing, that made it worse for me ). I struggled on with a old not-upgraded version of the kobo reader app for a while, then switched to bookari (Mantano as it was called then) and Moon + reader instead.

so probably for all that time my books were stored twice, once as single .epub files and also somewhere in a hidden app database, but that never bothered me. I was not trying to squeeze thousands of books onto the device. And having to do the "import" thing, once per book, was no big deal either.