Smaller internet forums, subreddits, Discord/Slack groups, etc tend to be a lot more civil than the likes of Twitter or YouTube are.
So a revival of those types of sites and communities will help a lot.
As will returning to the days of multi pseudonyms for different websites. Because people are not one sided. They don't always act the same way in every setting.
No, their behaviour depends on the company they're with. They might act one way with family, another way with friends, another way at work, etc.
That's how society stays together to some degree. People don't know how others act in other settings, and they don't care. Your coworkers likely have a whole mix of political opinions, but since it likely doesn't come up during work, it doesn't really matter.
Social networks seem to be trying to demolish this sense of separation between sides of people's personalities, and that's making society more and more fragile, as one wrong move means someone's entire life gets destroyed by the internet mob.
Oh, and decent moderation too. Unfortunately for Facebook and co, you can't automate moderation and expect it to work well, and you can't outsource it to a bunch of full time employees in a distant office somewhere. It has to be done by people with a real investment in the community, which is again where a well run small community shines.