Since I’ve only collected the revised content, I’ve picked up the alt-art starter decks, which are made exclusively from OOP cards. I’ve also added the "Core Card Alt Arts and Other Gorgeousness", as well as the "2023 Art Hero Cards" and "2023 Art Player Cards". These help fill in a lot of out-of-print cards with minimal overlap and are still "official" cards, just with different art on them.

I’ve been meaning to check out the ALeP (A Long-extended Party) content. It's a fan-led continuation of the game after FFG stopped publishing. It adds new player and hero cards, along with fully developed campaigns. Supposedly, it's very good, but there’s no rush to dive into it if you haven’t gone through the official content yet. You'll still be able to get the cards printed after 30 June, just at a higher price, so nothing there is technically "rare".

TL;DR: Here’s a buyer’s guide:
https://docs.google.com/spreadsheets/d/1guHEko4uw47aGcJvaNK0hT3smyX1EsxNGSucT2NVorQ/edit?gid=0#gid=0