They sometimes list their socials (LinkedIn, Twitter, personal site), not sure if you tried those avenues.

> self-promoting on relevant subreddits, but my Reddit account got suspended shortly after

That's very odd. People rarely flag FOSS projects. What did you do specifically?

> other outreach ideas or low-friction ways to ask for feedback without sounding spammy

Try to find people using similar projects as yours, ask them for feedback. Write blog posts where you genuinely add value, share them in the relevant communities.

You may want to expand and not rely just on Discord. Any such platform acts as a filter. There are people who simply don't use any given social media outlet at all, and they aren't likely to sign up for a specific one just to talk with you.