JFC, that's not how the installation and the first run of Calibre works at all. I'd write "I'am not sure why you think people would think calibre is first a manager library and not a book reader after installing it" but I'd be lying.
I've used https://www.kavitareader.com/ but it's not perfect either, this space could be "disrupted" by a Plex like product with opensource + licensed offerings but the target market is pretty small I think.
I'm not sure what the container is for...
Can someone please tell me the diskspace requirement for Foliate?
However, it needs flatpak runtime. If you already have different flatpak apps, this is not an issue, it is going to be shared with them, but if this is going to be your first-and-only flatpak app, you need to add few hundreds MB.
I'm pretty sure I already have flatpak, and enough space to run both Calibre and Foliate to compare them.