I must start by stating that I have never played on Argent Dawn, so I can’t shed any light on that.
I have all 40-something of myvroleplaying characters on the connected RP realms Moonglade / The Sha’tar / Steamwheedle Cartel.
These realms combined have less players than Argent Dawn, so if you want constant abundant roleplay, don’t come here 
However, some of the players here have transfered from Argent Dawn because Argent Dawn was a bit… much.
If you get a character on Sha’glade Cartel / SMT in an active guild, I’m sure you’ll have all the roleplay you want; but also (and more importantly) generally nice people who will help you out as a beginner 
Outside of guilds, you can type /join LookingForRP in your chat window to join a chat channel called LookingForRP (on Sha’glade Cartel, I’m not sure this exists on other realms). Here you can press Shift and O simultaneously to see who’s in the channel, and start chatting or asking questions:)
Then there’s the BattleTag LookingForRP
(which Blizzard won’t let me link from my phone, because everyone knows Blizzard doesn’t like phones, and why would they want a Blizzard link in the official Blizzard forum…)
which works regardless of character, faction and realm, letting BattleTags chat either in-game or in the Social tab of the Battlenet desktop app.
Oh, and for a beginner, I’d recommend MyRolePlay (MRP) instead of TRP. MRP is much simpler and easier to use.
I believe both add-ons are available for download on CurseForge, whichever you choose.
(You can download both and try them out, but I don’t think it’s possible to run both simultaneously.)
I think you’ll see my point about MRP - unless you want the abundance of options and details that TRP3 offers; which is fine, the important thing is…
If you want to roleplay, give it a try 
If you want to do it on SMT; welcome!
And if you don’t choose SMT, that’s fine, too. The (other) important thing isn’t where you roleplay, but that your roleplaying entertains you 
Last tip for now: Don’t transfer all your 48 characters at once
Start with one, on the realm you want to try, and see how you like it 