I got some users. What's the best way to gather feedback on if they like the product/does it solve a problem?

I launched an iOS app, Freetime, late last year and have iterated on it a few times since then. I've been able to get over 400 registered users (not monthly active, but at least downloaded and signed-up) by word of mouth, reddit, PH, & Indie Hackers. That may not be a lot to most, but is to me.

I'm at the point in my new product lifecycle where I'd like to figure out if I've got something. I'd like to get feedback from my users to see if this concept is worth continuing. Are they finding it valuable or not? Am I solving a problem for them? Am I solving it well? What do they like/not like?

The goal is to figure out if I should double down and continue. For those who have been through this before, especially launching an app, how did you gather feedback? What sorts of questions did you ask?

Note, I have around 120 emails from registered users. I don't want to feel spammy, but I'd like to get some honest feedback and don't know how else to do it then email.