Cursor's planning functionality is very similar and I have found that I can even use "cheap" models like their Composer-1 and get great results in the planning phase, and then turn on Sonnet or Opus to actually produce the plan. 90% of the stuff I need to argue about is during the planning phase, so I save a ton of tokens and rework just making a really good spec.

It turns out that Waterfall was always the correct method, it's just really slow ;)