Maybe, if we could join in-progress lobbies with a spectator mode, and then join the match when a new round begins, we would see more full lobbies more quickly ?

Yes, this is what we hope to be the case in the future; atleast for racing. Unfortunately blocked by the SpectatorModule rotting in the absence of Private Matches the past couple years. I’ve slowly been progressing the refactor in my spare time but the PU is our top priority so couldn’t say when it might be ready.