I can't recall where exactly I saw OpenZiti, it was probably from some discussion on HN. I went to look at it and thought it looked promising, but I'm already partway down the path of Nebula, and as you say it's partly because of Nebula having Slack experience behind it, partly because I have some experience with it, partly I like the architecture. I'll take a peek at OpenZiti again and refresh my understanding of it.