Thanks,
I decided to try a Samsung EVO Plus 32GB. It actually has pretty speedy 4K read/write speeds. I'm thinking of making the upgrade more for the operating system, returning search results, sideloading transfer speeds, opening books, etc, rather than the sheer number of books stored.
In case anyone wants to look at some reviews of cards, I found one site with a nice comparison.
https://www.cameramemoryspeed.com/re...icro-sd-cards/
And here is the site that gave me info on the Samsung Evo Plus.
https://www.jeffgeerling.com/blog/20...omparison-2018