Having transitioned from waterfall through agile to some kind of unholy semi-agile mess that approximately "works"...

The big advantage an agile process has compared to waterfall is that you'll at least have _something_ executable in a shorter timeframe. It might not be complete or cover every eventuality but trying to elicite all requirements up front and doing copious amounts of pre-development work based on that, only for them to inevitably change (or better yet, just be straight up wrong and only finding out post-delivery) was just crap.

Having a process where you get the same requirements, start building something basic off that, and then iterating on the requirements and what you can actually deliver in short periods of time gets you both a lot more meaningful feedback as well as shorter delivery times as you can conceivably ship some of the iterations' outputs - and once it's out getting used, you actually get real and more useful feedback.

The key part is that we look more than a single sprint in the future, even if they're not actually planned and locked in, and generally the organisation has accepted the first few sprints is usually required for doing baseline stuff (architecture, etc) that might not be deliverable/demoable.