When planning deteriorates into elaborate guesswork, we end up looking at risk in terms of upside and all the things we need to get right.
Instead of what we need to get right to head down a specific path, think about what you need to know to determine if the current course makes sense. What will this information cost to get? Can you afford to get it? Can you afford not to get it?
When planning is a worthwhile investment of your time, you'll notice you're spending more of it in the real world than writing documents.