It's funny but due my career I found out that most efficient way to know if people carrying about smth in a product - is to break it. If they soon stating to write you - they really care about it
One-on-one communication and online surveys after product delivery.
Although many users may not provide feedback, as long as the sample size is large enough, you will gather valuable information.
We do a lot of surveys, user interviews and user tests with existing users, but to find external participants, we user paid tools that recruit respondents for us