Change existing WordPress Multisite main site to subfolder

Question

I want to change my existing WordPress Multisite main site to be in a folder.

For example, this is my current main site of the network: example.com.

Other domains in the network have folders like example.com/it/ and example.com/es/

I now want the main site to have a folder as well. Like example.com/en/.

How can I do that without adding a new site or add a lot of redirects?

This is what I came up with so far but I don't like the idea of so many redirects:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\.)?example\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/en/
RewriteRule ^(.*)$ /de/$1 [L,R=301]

Also I need to add an extra site for that and leave the main site "hidden" in the network.

Is there another/better way?