Companies do share data either directly or indirectly, so I don’t think your approach of spreading it out is as effective as you think it is. Still not the worst thing to do in addition to other measures though. Another thing is to use a unique email address for each site to make that one common data point no longer common.
I think you could list the instances/communities its been posted to in order of: ones you’re subbed to, ones in your instance, and finally the rest. All in alphabetical order. Obviously would want some options to change aspects to fit people’s preferences/needs.