You can't ask people to join small servers that have the biggest risk of shutting down without creating migration toola thst migrate all the content along the likes and comments

• Institutional servers (schools/universities running servers for faculty and students, companies running servers for their own employees)

This is the best long term strategy. News orgs should be hosting their own Mastodon instances at the very least. Same with schools and government.

It solves a number of problems - for them. So many news organizations and government offices are reliant on Xitter. That means that they are at the mercy of the owner of the platform for their messages to the public. Hosting their own instance puts them in charge. They can get out messages reliably and the public can trust that they are who they say... Just like an email address or URL.

Schools pay lots of money to private corporations to run bespoke university messaging systems, and are likewise reliant on those companies to provide administrative services such as moderating. Moving those communications in-house will be cheaper and simpler.

We should all be pressuring schools and local governments to adopt these technologies.

Let’s get rid of open registration instances

How?

Nobody is stopping any of your bullet points from happening. Those are all options today. Any one of those groups can spin up an instance and nobody is going to stop them. Some already have

But isn't the idea of forcing someone to (not) run their own server however they want antithetical to the whole concept of the fediverse?

You can defederate your personal server from open registration servers if you want. But you can't "get rid of open registration instances." That's just stupid.