So I just tried this out and was able to access my Start9 admin panel with the Tailscale-provided IP. But, once I click on the LAUNCH UI button in Nextcloud, I’m taken to the .local address (which of course doesn’t work if I’m not on my local network). Am I missing something?

hm, Nextcloud has a local link hardcoded in their button (lol) or else there may be a Nextcloud setting that makes it sensitive to this environment difference? I don't know enough about Nexrcloud, but this is definitely their UIs fault. do you know what path that button is supposed to bring you to? might be able to navigate to it directly at the tailscale URL?