Would it be possible to create a read-only instance that was federated with every single other instance? Possibly there is already one or it's been tried and didn't work but the way I envision it is this -

You sign up to the read-only instance and can see the entire fediverse's contents and posts and when/if you wanted to comment, the app would let you know that you had to sign up to the instance the comment came from to post your own comment and take you that instance's sign-up page.

One of the biggest turn-offs for me was finding which ones to join and/or had content I was or might be interested in.