You know, there is a saying "those who don't risk, don't drink champagne". Unless, people from Aarin would make the Discord server you won't know how it will go. So, it is kinda pointless to make guesses and write on water by using fork.

But let me tell you a thing as an owner of Discord server. Even if server gathers enough people but if no one with interact with it, it won't live enough.
So, at least having 1-4 active users daily could be enough.

Yeah, you can organize stuff nicely by using threads like introduction one, then non-bl interests (manga, anime, games, dramas and etc.), and of course BL counterpart to it along general section which could be served as chat, then, suggestions/FAQ, the rules and etc.

On the other hand, there is also a thing like people wanting to stay on dead bird's site or other popular social media 'cos "our friends and Japanese/Korean artists are there!". So, that, of course, would affect Discord server's activity as well.

More so, don't forget that some people are either too shy to interact and prefer how they call "to silently admire". So, when one considers making server, you need to note that as well 'cos majority of users would be either too busy IRL or straightforward shy/too introverted.

Of course, Aarin forum and Discord server could work in theory as having symbiotic relationship. Like, you can announce forum's events on the server (there is special option for that).

But that is just a personal opinion from Discord server's owner... So, take it for a grain of salt as how people say.