That doesn't matter. If your followers want to support you, they do so independent of whether the content on your website is exclusive or not. It would be nice for you to give early access to your members, but that's up to you. What is important is that you must hold the usage rights over the content you offer on Steady.
What if my content is already published elsewhere?
Non ha importanza – chi ti sostiene paga per il tuo lavoro, non per l'esclusività