Better than Material Files for SFTP but it reprompts for the SMTP password inconsistently on app switch. Kind of annoying but whatever.

I don’t personally read manga but can’t you source your books from anywhere and just drag and drop then onto the device? That’s what I do for normal comics and ebooks with CBR files for comics and EPUB files for other books.

I have bought a book or two on the Kobo store because I wanted access to the book the day it came out vs. waiting to source it via “other means” and I was able to just drag and drop off the Kobo that book to my computer and other devices for reading elsewhere. Granted this all depends on what encryption if any the store is using.

But also it goes without saying that any alternative store would / could have their own encryption and thus make your purchase disappear if it goes defunct as you state.