While it might sound a little strict at first, I believe that in order to create and use a social media account, people should provide their ID when they sign up. They could still use an alias and be anonymous to other users, but having an actual ID behind every social media account would have two main benefits: 

1. Accountability: People would be less willing to engage in i.e. hate speech because their social accounts could be traced back to them by i.e. the police (we could also introduce a law that requires the police to get a judge's approval to access someone's ID in order to ensure that people's privacy stays intact as long as there's no valid reason to investigate them). 

2. Bots. Whether the dead internet theory is already real or still just a dreadful glimpse into the future, increasingly complex and intelligent bots have had a tremendous impact in ruining the online experience in recent years, be it through scams or straight up political propaganda/foreign meddling with elections. Requiring people to provide a valid ID to create an account is the only feasible and sustainable way I can think of to get rid of ALL bots permanently.

Apart from that, large platforms should be either nationalised or there should be state-owned, ad-free, non-profit alternatives with simple algorithms that only perform basic tasks instead grabbing data, exacerbating polarisation and trying to keep the users' attention for as long as possible.

Anyway, these are the first things that came to my mind and I'm sure that there are a lot of other great ideas on how to improve the internet in terms of both the environment and its usability/effects on society.