It's your proposed solution that's the problem. The answer isn't to make Discord public, it's to convince people to move off of it, and quite honestly, if you want people to leave Discord so badly, you'd be better off setting up separate public forums for the open source projects you are interested in on your own and convincing/bribing respected members of the Discord to post there, or copy/paste technical info there.
I feel the same way about Lemmy so I sympathize with you, honestly.