Also, the new editor is missing more features: tags, categories.
See, it’s not, actually. Things like tags and categories being missing are telling us that you have a problem on your server, where something is blocking the REST API.
WordPress has a thing called the REST API in it. It’s basically calls to the site that have the word “wp-json” in them. It is how the new editor talks to the site, in the background. The REST API has been around for a couple years, but this is one of the first major uses of it.
Now, to display the categories, for example, the editor sends a request to the site that basically says “hey, give me a list of the categories”. If you are blocking that somehow, you don’t get the category listing.
How are you blocking it? Well, over the past two years, a lot of “security” plugins and other misguided tutorials have recommended to either block or disable the REST API. They were wrong, and you need to not do that.
If you have any security plugins, turn them off. If you have any rules in the .htaccess file that block the wp-json requests, remove them. The editor in your browser needs to be able to talk to the site, to display things like tags and categories, as well as to be able to send the post content to the server for publishing. So you have to not block the REST API for it to do that.