Backing up books when my iPad is outdated, and did not get the books from Apple

Hi- spent all day trying to figure out why I couldn’t update my iPad, only to learn that the Mini 4 model is no longer supported. ( I have version 15-ish, and can’t get more current than that). I have about 2000 books on there, and never used the cloud before. Is there anything I can do? Can I send books from my iPad to my iPhone, then upgrade my cloud storage, then backup? It seems crummy that I can’t do this by simply backing up the device itself…. Thanks in advance!