WrA is a perfectly fine server for an Alliance character as long as crowded public RP areas aren’t a must-have for you. WrA does indeed have Alliance guilds, as well as a long-standing tradition of cross-faction RP. You also don’t need to find a guild right away in order to check things out and meet people; WrA features multiple public, scheduled RP events per week that you do not need to belong to a particular guild to attend.
If you decide to check out WrA, I recommend joining the Wyrmrest Community Project Discord server. You’ll find a lot of WrA’s events listed on their calendar, and you can network with other RPers there. WrA does have opportunities for walk-up RP, but it helps to be able to ask where people are and/or invite them to come to you.
All the WoW RP-related Discord servers I’ve ever been in have either been entirely OOC or had both OOC and IC text channels in them. I’ve never personally been in a guild Discord or other Discord community where people RPed in voice chat, though there may be some out there.
I don’t know anything to speak of about Moon Guard, but it seems like they have a guild RP and campaign RP presence as well. Nobody can really tell you which is ‘better’ for you personally, though – it’s a matter of personal preference and of what kind of personal experience you end up having. Bowd’s suggestion of making characters on both realms is a good one.