if the server is *nix, I use this script on linux to convert to .webp my images from time to time, in a lower quality (80%) but still almost equal visually. It helps on about halving the used space, but that may vary. A few tweaks and it may be usable here too (no need to update posts img links, just create a symlink to the webp image with the original image name, after removing it).
EDIT: oh btw, I would like that "download my posts" could include all bookmarked posts/threads, that would actually be helpful/useful!
EDIT: also, it may sound fun, but... very old pictures of low visualisations threads, could have even more lowered quality, like 50% prior to a possible purging, I mean, something barely but sufficiently/usefully visible.