For a new product in a new market, I don’t think you can ever validate the problem space enough. Once you have reasonable confidence that the users (and buyers) exist for the problem space your product tackles, and there are viable ways to create a business out of that solution, the best next step is to involve engineering to build a prototype of the solution. Then you can test that prototype with a representative sample of the expected user base to get early feedback and iterate on building out the product and grow the business.
For 0-1 products, involving engineering early as a partner to the overall product definition and development process is best, so you can iterate together on building the right product for the right audience.