Moderation wise, it works. Business-wise, not so much.

paying a monthly fee for a user on a Mastodon instance.

That is exactly how it works. One membership = one account for Mastodon/Lemmy/Matrix/Funkwhale

having the user-generated internet hidden behind a paywall will not, for me, ever be an acceptable solution.

But it's not paywalled! The websites are visible for anyone. Anyone can follow Communick users. The only thing that is different is that people that sign up for Communick are paying from the get-go for the service they are being provided, like any other utility.