We also send (often a different) survey email (along with product updates) to folks who've subscribed in case we expect to learn something else than from the responses in the telegram group. In our experience, with our audience, emails don't work as well as Telegram.
What we also do to gauge user-interest for an Android (userspace) DoH client and firewall we build is:
1a. Include a link to Telegram community in the about page prominently.
1b. After every update, shows a changelog to the user with a button "suggest features" that emails feedback.
2. Contact email features on all our webpages.
3. Google the past 24 hours for keywords related to our app to see who's discussing it, and if it is a forum like reddit, leave a comment and personal email (as opposed to generic support at domain dot com) encouraging the user to contact.
4. Newsletter to collect potential customer email-ids. As mentioned above, survey are always sent out to them once in a blue-moon and only with accompanying significant product updates.
5. All incoming to generic mailbox are replied-to from a personal email. Emails always end with a remark asking the user to not shy away from sending bug reports, queries, and suggestions.
6. Monitor related subreddits where similar products/apps are discussed, and post replies only after the thread is off the front-page to avoid derailing a hot discussion with self-promotion (which gets you banned on most subreddits).