Currently, if someone links to another threadiverse post in their post, it is seemingly handled as any other URL is and opened using the in-app browser. Instead, I suggest that such links be opened as posts directly, with the user having the option to tap the back button at the top left to return to the original post.
Understandable about proxies; in regards to i.redd.it, I wonder if the issue in fetching images is the same one supposedly encountered by yt-dlp here?
Per this Reddit thread, it seems that Reddit has intentionally adjusted media hosting links to make it more difficult to load them outside of Reddit itself, but maybe something there would give a clue as to how to fix it in the same manner Mlem does?