A simple question you might be able to answer.
Does it matter to have the following setup
Is this SEO relevant or in any other important way relevant which domain is the main domain for a site.
The point is the following:
If it isn't relevant at all we could use an entry domain for the main site which would be also the site 1 and which would be actually only existing in the backend and has only a coming soon in the frontend - simply to administer all sites, do updates etc.
All other sites would have in the table also numbers - while the entry page would have all tables without any number.
While moving sites from one site to another it would be easy to concentrate simply on one number and all multisite management would be located in the site which has no real frontend.
If it matters what is the main domain the above setting would be somewhat contra productive, so let's wait for your answer and solution.