I am glad you brought section 230 up. Let's stick with Dominion example. You see the devil is in the details. They aren't liable for the speech per say. But what happens is they get sued so they can be subpoenaed to reveal the real identity of the people spreading defamatory content which causes Twitter to spend a lot of money as they often have to produce records of related accounts and so on.
In the past whenever a user caused the problem they had a set of rules to go by so they could suspend the accounts that are likely to get them hauled into court. Twitter can also be sued under theories that they are monetizing/profiting from illegal or unlawful content. What do you think happened to Backpage?
Then you have all of their upstream providers to worry about. If any of Twitter's upstream providers have terms of use that are more restrictive than their own, they have to comply with those terms if they want to keep using that upstream provider.