I mean… It’s decoding into garbage because you’re feeding it more than just the base64 section. I suppose if you’re already running nginx or something you could easily make a page that uses javascript to break the link down (possibly using /, ?, = as separators) and decode sections that look like base64. If you make it javascript and client side there’s not really any privacy concerns.

EDIT: Oops. My Lemmy client didn’t load the other replies at first, I didn’t realize you already had plenty of other options.

Also check to make sure the mobile browsers aren’t set to HTTPS only, or at least have an exception for that ip. I’ve seen that before several times.