I, too, would like to know if there are any good spots for Rune downloads that I'm not aware of. It seems like Rune mods are one of those things that's sadly eroding away from the net.
Here are a couple places I'm aware of, though they may not be quite what you're looking for:
https://www.moddb.com/games/rune (ModDB page, seems mostly to have co-op sets)
https://ettingrinder.youfailit.net/rune-addons.html (Single-player campaign reviews with downloads, I don't think there's anything there that wasn't on Rune-Library when it was active)
Edit: Went searching again and found a couple more pages:
https://en.ds-servers.com/gf/rune-halls-of-valhalla/maps-levels-missions/ (Seems to have a sloppily-implemented download cooldown timer, if you find the downloads are inexplicably not working, wait a while between each and try again)
https://sites.google.com/site/jorgeoscuro/runeleveldesigns (An author's personal page with a couple of maps)
Edit again:
Look here too, there's additional stuff archived including maps for multiplayer modes:
https://unrealarchive.org/maps/rune/index.html