For my Kobo I’ve mostly used Anna’s Archive and then converted epub from there into kepub format with Calibre; just need to rename the file to the epub file extension afterward (I usually keep both like My book.kepub.epub for my own reference).

Edit: MyAnonaMouse is indeed a great community as others have said though I found their selection to not be as good though that might be biased to the topics I prefer to read.

Same for me: I read on both laptop and phone so sync is important