How to know people are willing to pay for your product?

Oh wow, nice work!! Seems like you we're already ahead of the advice ! (either that or you have a time machine haha).

Your original post was about having contact information so you could reach out and speak to people about your pro product. So let's assume this product doesn't exist yet, which is the reason why you want to speak to customers, or you have some ideas, but you don't know which ones to implement first.

First, I would split test a few ideas. Here are just some suggestions, but ultimately it doesn't matter what you start with, as long as you split test them, gather the data, then take action based on what the data tells you.

GENERATE INTERVIEWS

1. Send a 'thank you for completing my survey' email to 20% of the group. Tell them you're always looking to improve your product, and would love to speak to them to find out more about how they use it. Provide a calendly link as call to action. Or have some strong call to action.
2. Send a 'I have a new product' email to 20% of the group. Provide them top level information about what the new features will be, and have a call to action to sign up page for early access with a discount code
3. Send a 'I'm building a new pro version and would love to you to vote on features' email to 20%, and have a call to action to a survey to vote for the feature you would most like to see in the new product. You can use something like typeform (or the tool you already used) to capture the survey data.
4. another idea for 20%
5. another idea for 20%
   etc. etc. etc.

I would recommend always leaving a "I would love to to talk to you about how you use the product" type call to action on all of these, as being able to speak to users to run interviews will be super useful in shaping your new product. Don't run these all at once. Start with one, get everything in place working, and then you can run a a couple at a time. Time-box the experiments, play with different times of day to deliver the emails etc. Try not to change to many variables at the same time. Use a tool such as mail-chimp so you can organise these as separate campaigns, and track the results.

Analyse the results of the experiments, work out what worked best, formulate new experiments, execute. Repeat and continue to optimise.

Set up a calendly link so that your users can book in 15-20 min slots with you so you can conduct interviews with them.

INTERVIEWS
Not sure if you have read running lean, but there is a Problem Interview script in the book. Since writing that book Ash Maurya has updated the script to use a new format based on customer forces.

Customer forces is a bit harder to understand and conduct interviews with, but is much much more powerful. Start where you feel comfortable. There isn't a lot of information available on the customer forces, so if you struggle with it I'd be more than happy to jump on a call with you to go through it.

Here's a link to the original Problem interview script:
https://startitup.co/guides/285/problem-interview-script

Here is a link to the Customer forces script:
https://blog.leanstack.com/the-updated-problem-interview-script-and-a-new-canvas-1e43ff267a5d

I definitely recommend signing up to LeanStack.com, there is a lot of awesome content on there which is super useful for understanding customers and validation. Plus its all free for the first couple of playbooks.

If you need anything else I'd be more than happy to help (no sales, just helping like-minded people). Here's my calendly, if the times don't work just let me know and I can see what I can do:
https://calendly.com/startup-success/lean-startup-1-1

Good luck!

Just an idea, if you have noticed the pattern of downloads (eg., mostly on any weekday there will be 200+ downloads), try changing it to free trial during weekdays for a week or two. This approach may probably give you some directions.