I feel you are pointing in the right direction, but you did miss some stuff that is commonly missed. (I am going to preface that all I am doing is presenting facts, corps can burn in hell for my personal opinion)

• Freedom of Speech only has a bearing on law, government, and the agents thereof. Corporations in the US are not bound by the Constitution, only the government. Corporations and individuals operating a space where the public are able to act are bound by the laws, but as long as they don't directly violate any if those law, they can restrict speech as much as they like.
• While echo chambers are a major issue, and one we should all be focused on making sure we don't get trapped in, they are not the largest issue concerning the issue at hand. Humans are more prone to engage with controversial topics, whether that is surging to the protection of something that affirms our biases, or lashing out against things that offend them. Social media platforms only care about so-called "engagement". Their statistical validity for investors and advertisers are strictly based on sanitized numbers regarding how many users live on their platforms, how often they post, and even more so how often they comment. Polarizing posts see the most commentary, so social media companies are financially incentivized to propagate as much polarizing information as possible, regardless of the content. The advertisers never see what the post info is or how how much hate and vitriol are in the comments, and they don't care (some are starting to realize). All they want to know is "if I pay you to put my add on peoples posts, how many people will see them?". It is disgusting, but true. Bad news sells. Tragedy sells. Hate sells. Polarization sells. It makes me long for the days when all we had to worry about being manipulated by marketers with was sex.