I think the best source of feedback currently is just playing with friends + playtesting. (Also keep an eye on your island in the days after you release it, you’ll be able to play with random people if your island is featured on one of the discover rows.)
Hopefully an in-game feedback system will be implemented eventually. I’d also love to get a notification when my island gets put on a row.
I’ve found the Discord servers to be strict about self-promotion. (Only specific places on server to share your island code)
I encourage you to try asking questions here on the Forums - I think there is less of a chance for your question to get ‘buried’ here when comparing to Discord.
As for servers, I don’t know any specific ones - but I’d look to see if people who are creating UEFN tutorial content have servers.