Welcome to the community, @Abraham_Davidson! :grinning_face_with_big_eyes: First off, you mentioned “the newly added functionality of logic on lookups / rollups etc”, but none of those field types are accessible via forms, or interactively driven by what a user does with a form. Those only come into play once the data from the form is added to the base.
From what I read, you want a system that will dynamically present a culled list based on user selections in a form. However, the new logic available in forms—similar to the logic added for certain field types—only affects field visibility when certain conditions are met. What you want is something that dynamically alters the choices available in Field B when a certain option is chosen in Field A, and that type of control still isn’t available natively in Airtable. For that, you’ll need to use a third-party form setup like the On2Air: Forms system built by @openside.