I wanted to start selling access to the digital version
I am not sure what you are trying to sell. A downloadable file or access to a website or access to a server process running on your site.
I am currently assuming you have a host that runs that digital version and a buyer would be able to connect to that server and distribute invitation links to employes, so they can all connect on that website and no installation of games or Steam clients or accounts on gaming platforms needed.
If that is so, just sell the licence key on your website - in any configuration you need. You do not need Steam or Itch for that. And maybe upgrade your server to https someday.