Try Guild Wars 2.

You can play basically the entire game solo if you want. It would take you years to play all the solo material. They have mass battles to take down huge bosses. You can literally join without talking to anyone. As long as you cause at least 1 hit point of damage you get the chest at the end of the battle.

You don't have to do those mass battles (meta events) if you don't want to.

My guild has a discord server but no one is required to use it. You can use the in game text chat if you want. If you want you literally don't even have to talk to anyone.

There are multi players areas, but I just spoke to someone who played for 9 years only doing player vs enemy. He just started player vs player.

If at some point you want to use discord someone will help you, but again no requirement.

Just to get the mount called skyscale (if you wanted to go nuts could be done in a week) I've been working on it for a couple months now. Like I said there is a TON to do in that game.

Game is pc only it's about eleven years old.