Nice to meet you! I look forward to reading more of your work.
I'll answer a few questions. I put my best writing on Medium when that writing happens to be essays, reflections, poems, or short fiction. (I'm an academic and that writing goes elsewhere.) I think Medium is not proprietary; you can publish anything on Medium anywhere else later. I now have all my writing behind the paywall, but I'm rethinking that decision. And I submit 3-4 pieces a month, and I have no intention to submit more often than that.
All best to you!