Landing pages, surveys, waitlists don't work anymore. Coding is a necessary part of validation. Give them something to touch and play with and they'll give you more accurate feedback. Hackathon style coding is faster too. Make disposable prototypes.
The best successes I've worked with have been
1. A keto recipe app with hardcoded recipes made in two weeks. Users told me the way I displayed it was confusing and that it needed carb counts. I did the fixes and they started throwing money at me.
2. A social media app for complaints. Turns out people wanted social media for a small area, e.g. everyone in an apartment area. It ended up being a visitor management/appointment kind of app with social media features.