> GitHub doesn’t have a DM feature and most people don’t list an email
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.