Not directly affiliated with Kart Krew or any other organized Kart community. Please don't bother other people about this being broken.
(this video plays audio, fair warning)
HyuuSeeker is a work-in-progress automated addon downloader for SRB2Kart, searching serveral open directories for the files you're missing. It's faster and more reliable than the in-game downloader, but similarly convenient (hopefully).
It currently has first-class support for the RDX server series, the :D? server series, WATCH SYMPHOGEAR, crapbox, and Dryft—these are the servers that have "open directory" style file indexes, instead of Google Drive or MEGA folders. It will work for other servers, but may not be able to find all files on its own; as more mirrors are added, this will get better.
Place it in your SRB2Kart folder and run it.
Support / suggestions on Discord (if you have issues, bother me here!)
Source on GitHub (if you're not a programmer, you don't want this)