Have you tried the built-in Books app on iOS? I find it to be a simple book reader that allows for ePub files to be opened from the Files app without having to sync them manually using something like a Mac.
I have not, I still consider myself a iOS newbie and tend to avoid the Apple software because of their lack of features and usefulness. But I'll give Books a try, didn't realize it let you read local files, thanks a lot for the recommendation! :)
In case others are in the same situation, I ended up doing something like this:
idevicepair pair
mkdir ~/iphone
ifuse --documents com.google.chrome.ios ~/iphone
cd ~/iphone
cp /home/user/books/*.epub .
cd ~
fusermount -u ~/iphone
If you’re on Linux, using iCloud to add files there should be easy enough as well. Then I just added them as I needed them. I’m on a Mac though so it’s a bit easier.
Sounds like you found a solution though.