I wish you guys luck, but if i'm being honest a lot of people aren't going to want to disclose some of this information even if it's privately to a single person. that being said giving a false profile to apply is always something that can be done. you're not going to get everyone to give you their ACTUAL steam info.
Hell some aren't even going to fake it, I know a few people on here just avoid steam like the plague, and they might just make a shell to join your group if they feel so inclined.
and the 21+ age limit seems so arbitrary it reminds me of American drinking laws. If your objective is to gate childish behavior out of the community why set the bar at 21? why not set the bar at a trial within the community to access the individual and see if they're going to mesh well within your structure. Also people are just going to lie if they're under this age and they want to be a part of this group that badly, and you'd be none the wiser unless you demanded some sort of hard proof, or started demanding everyone speak of microphone (Which is another thing that's going to gate a ton of people from ever wanting to join).
As for Discord, while I use it personally, and it is a useful platform to keep in touch with people, the amount of bullshit that Discord as a company pulls is beyond "not OK" for a pretty substantial group of people, and getting your community banned for some pretty silly reasons is also a good reason not to use them as a platform, as well as user data collection being so extensive it'd make the CIA blush.
In short, your requirements are going to gate a fair amount of players with the type of know-how you want from joining. I'd suggest spreading out to better platforms for communication, and probably removing some of these restrictions or otherwise retooling them to be more user friendly.
Good luck finding the type of people you want