Does it have to be Discord instead of Slack? Maybe it's a generational gap or something, but I feel like Slack is much less distracting to work with, in terms of having different workspaces (instead of being shared with someone's gamer name), not having a flood of huge animated stickers and emoji, etc. Every time I'm in Discord I get overwhelmed by the chaos, and Slack feels much more focused on just getting work done...
Doesn't have to, no. Discord had a much more generous free tier last time I checked.
What about the users who won't use/sign up with Discord?
In my experience a decent chunk won't, so the same info needs to be sent with email.