Are you sure your pieces rank high for everyone and not just for you? Keep in mind Google customizes search results…

The fact that repurposed viral pieces often tank for you just goes to show that what does well on one platform doesn't necessarily do well elsewhere. But it also means that what does terrible on one platform could do excellent elsewhere. So I wouldn’t just limit myself to repurposing viral Medium pieces.

As to where new pieces should go first, I'm still debating that. For now, I'll keep posting on Medium first since I still feel some sort of loyalty towards it and it's the platform where pretty much all my audience is.

J.J. Pryor, though, is experimenting with publishing on Substack first:

https://medium.com/feedium/toward-a-more-relational-j-j-b8eeb280f112