I should probably have said side-loading binder pages. Nothing special, just that you insert the cards using the longer side, which makes it easy to put them in / take them out (though they also fall out if the pockets get loose, which I quickly found out they do if you put more than 2 in each).

So I use something like this:

https://www.amazon.com/Ultimate-Guard-18-Pocket-Side-Loading-Binders/dp/B019I2DVO2

But different colors, blue for guardian cards, red for survivors, white for neutral, etc. They can hold a pair of cards in one slot, so when you go to upgrade, you just slide your chosen pair out, and slide in whatever you're removing from the deck.

And stuff them all in a regular office binder like this one:https://content.oppictures.com/Master_Images/Master_Variants/Variant_1500/630383.JPG

The rings open real easy with the press of a lever/button (not like the cheap simple ones where you have to basically pry them open with your fingers) and I can take out individual pages to browse faster or hand out to people.

But I have over 30 pages now and it's getting a bit unwieldy, so I'm thinking of splitting them by class into multiple albums, like this (but I haven't tried them out yet, idk if the cards are as easy to slide in and out as with the pages I use now):

https://fyft.cz/en/magic-the-gathering/102030-blackfire-12-pocket-premium-album-4251456402108.html