Hey all. I'm looking to create a subclass for warlock where at level 3, you choose a specific school of spell (enchantment, transmutation, etc) to specialize in, and immediately learn a cantrip, level 1 spell, and level 2 spell from that school of magic.

Further, at levels 5, 7, and 9, you would be able to choose a 3rd level spell, 4th level spell, and 5th level spell to learn respectively from your chosen specialty school of magic.

I'm getting stumped by how to set up the stacked choices: How do I set up this subclass feature so that a player can first select the school of magic they want to specialize in, and then select spells from that school's spell list to learn?

The "easiest" solution I can think of, unfortunately, is to make all of the schools separate subclass entities in DnDB. I know that's not exactly what you're going for, but you could maybe somehow key them together so a person knows they can take any of the options etc.