I can recommend https://www.spiralknights.com/ . It’s free to play. If you install it with Steam, you get voice chat with other steam players. Party size max in dungeons is 4. Parties can be public, invite only and open for friends.
Even tho there are unnecessary event items (mostly cosmetics/reskins/gambling boxes) that can only be bought initially with IRL money, those items can be traded for in-game currency afterwards, so everything can be obtained for free.
some people think the game imvolves lots of grinding, but it’s actually their choice to do that. you can just play for fun and still progress. there is even one guy who famously stopped playing dungeons and just trades stuff.
no, i just checked a package manager. it’s still available on ubuntu. it’s like i said “nvidia-settings”. check your package manager