I don’t know, but I’m just checking the documentation: Reviewing & Cramming - Obsidian Spaced Repetition
It seems like if you can’t choose “all decks” (and maybe you can), you can definitely choose all sub-decks. So at least it seems you can you put all your separate decks under a parent deck.
And cards can belong to multiple decks.
And this comment makes it seem like you can in fact choose “Random card from random deck”. [FEAT] New cards first? · Issue #610 · st3v3nmw/obsidian-spaced-repetition · GitHub