Have you read The Mom Test? Highly recommended.

Here's a summary: https://www.slideshare.net/xamde/summary-of-the-mom-test

The closer you are to the target user and target buyer (not necessarily the same person), the better. You can learn a lot from anyone, but it might be at a pretty abstract level and less useful.

Focus as much as you can on users and buyers. Speak with them often, observe what they do often, wherever they are - conversations with you, chatting with each other online, complaining about current pain points with existing solutions, including in reviews for products that solve similar problems.

Good luck!