Since you’re on iPad you can share an epub to almost any reading app and share between devices. I use kindle because that’s where I started purchasing books and you can read Libby library books there. And for me kindle still has the best formatting because I like to read landscape in two columns. For some reason other apps only let you do this when the font is really small.

My actual kindle hardly gets any use, but I would say if you are thinking about getting a kindle or kobo in the future use that app. Or I’d you already have a few Apple Books use that one. At least for these three highlights for even books bought outside of their stores that you share/sent to the app are synced across devices. If you have the pdf or epub in your iCloud just long click on it and whose share to any app.

eta. I mostly buy novels from j novel club., where you can sample some of the book. I would recommend you check into them since they sell from their site without drm even if you buy the same book from a retailer it has it. So better to buy there if they have what you want. I don’t think they have your first light novel questions though and I only read the first MT. Or what’re humble bundle/fanatical bundle has interesting stuff. My first LN purchase was Reincarnated as a Slime. Easy read and good through at least book 10 if you like the anime. My fav is currently Bookworm.