WhatsApp now has channels and works with just your phone number so it's very easy to set up even for people who don't already have it.
I don't really like the Discord plague... I'm already on Facebook, WhatsApp, and this forum for instance, why do I want yet another app to communicate with?? I said the same about WhatsApp to start with, Facebook messenger is perfectly adequate.
It comes down to the same thing - what do most members already have access to? Use that. If there's no consensus, pick the easiest thing which is likely to have longevity - which is almost certainly going to be the most popular (in terms of global users) thing... so WhatsApp, Facebook, or, sadly, Discord...
I wouldn't bother with your own forum or any obscure apps, it'll take time to manage and might simply stop working or cease to exist.