Thank you both very much for your suggestions and ideas!
I’m giving up on this for now; I’ve been working on it for far too long. I thought it wouldn’t be that difficult, but now I’m a little lost with all the proxy topics like SOCKS and SOCKS5. I assumed it would be as simple as adding a few parameters or configurations to the env section of app.yml, but it seems I’d need to invest many more hours in a custom solution. Because in general Im having trouble with getting a connection with proxy outbound to for example https://check.torproject.org/api/ip with curl. So I think I’d had to deep dive much more into proxying.
It would have been nice if I could hide the host IP in all the ways I know it can be leaked, but I guess that’s the trade-off for using such a powerful forum platform like Discourse. Discourse has so many configuration options; would it be possible to request a feature that prevents it from preloading links and previews or, more generally, from fetching external resources provided by users? I couldn’t find a request sub-forum here 
Yes, but I’d like the proxy configuration to persist after updating or rebuilding Discourse. Working with Docker can be a bit tricky.