I'm using a homebrew subclass of a sorcerer, but I can't prepare the majority of the known spells (including default sorcerer spells). This excludes cantrips, which I can learn any 5 of, no problem.

If you homebrewed the subblass yourself, go back into the editor and make sure that you did NOT check the "spellcasting" checkbox and the related fields.  Those are intended for subclasses of non-casting base classes (like the Eldritch Knight and the Arcane Trickster), not for subclasses of casting classes.  Checking those boxes for subclasses of a casting class has caused problems with spell selection.

That was the problem, thank you! Is there a resource I can use to make sure I did everything else right?

The only one I know of is the initial description of the homebrew subclasses.  It can be found on the changelog page (which you can also access from the little circle with a pencil inside near the top left of the screen).  It is pretty bare bones.  There's been some talk about more complete instructions/user guides being written eventually, but I think that's a ways down the road.