How to adjust text’s width and put it at center of post (Visual) editor?

One option: switch to the Block Editor instead of the Classic Editor. The Block Editor comes with any version of WordPress 5.0 or higher, so update your install, and then if you have the Classic Editor plugin enabled, disable it.

If you have more than one post type, you may want to look into the Gutenberg Ramp plugin, which allows you to choose which post types to use the Block Editor on. You might try just one post type to see how well it works for you.

In any case, the Block Editor has a “Fullscreen Mode” that completely hides the top, left, and right toolbars, and which also has a fixed-width middle column for all of your content. It’s very similar to what you’re trying to achieve in the Classic Editor.