If I recall, this is the SRD explicilty. Only 1 subclass per class.Another option might be for you to pick a single subclass for each class and just say "All Fighters are Champions, all Barbarians are Berserkers" etc. This would save you a lot of work and your players a lot of analysis paralysis but still simplify the game a bit.
Not sure I'm looking to simplify, although that's certainly an admirable goal, and for some groups the right choice (I'm thinking of one of my own groups tbh).
But for me, I was thinking about giving my players a bit more flexibility around their builds.

