[Quoth]

I've been using Lithium on Android, largely because Calibre and Lithium will see the same files and it works with both separate and integrated SD card mounting.

I was using something else, but an updated version required logging into Adobe before doing ANYTHING.

I was using PocketBook, also for PDF, but it IMPORTS the files, not sensible with huge amount of PDFs and Calibre has no idea what books are in the Pocketbook library.
I use Xodo for PDFs on the tablet now because I can copy the structure of nearly 200 Gbyte of laptop directories and it can browse the 256G byte SDXC card (£37). The XC cards shouldn't be reformatted without special tools. They use a different Partition type.
Also Xodo has tabs per document open. Most of my PDFs are reference materials. Some are old magasines or technical books in the Public Domain.

I only have PDFs that would work as A5 or smaller on the Kobo Libra. One or two old novels and mostly operation/user manuals for gadgets with insane UI.