Given the enormous scope of some of these projects I wonder if a prototype phase would help. Humans seem to have a hard time with conceptualizing and a slow, barely/non working and unscalable prototype might be worth the added cost/time. If it’s part of the requirement gathering process with explicit expectations set that it’s a prototype would it facilitate better requirements input? Naturally you’d have some people making assumptions about its fitness as a final product or MVP but (if a UI is involved) giant bright warning banners plastered all over might help with that.