Matrix/Element seems the best drop-in replacement for Discord. Can set up chatrooms in Discord "server"-style, or as persistent DMs with others. In the chatrooms, you can make text chat spaces and call spaces. No dedicated voice chat space, you just dont turn on webcam for the call spaces. Pretty sure it uses same voip tech. Selfhostable if you want. Encrypted. Really it is just a barebones but functional Discord. Matrix can be bridged to Discord as well so you can talk between either, should help ease the transition period for anyone jumping ship.
I like matrix and use it but it is no where near a Discord replacement. Setting up the modern Matrix auth and call are not easy and its just not as good or easy as Discord.
These won't run Crysis, but they don't need to.