Just wondering if I'd be missing out on anything big plot-wise if I cut my half-finished run through of The Legend short and switched straight to the reputedly more balanced Crossworlds?

I wouldn't do that, but you're told, more or less, what happened during The Legend, so you shouldn't be missing all that much. As far as balance goes, some op tactics from The Legend no longer work, but a lot of other op stuff is introduced so I wouldn't necessary say that is more balanced. Both games can be extemely easy at times and relatively tough during some battles.

Just remember to keep your save game folder in case you regret it.

Personally I'm one of the few that preferred The Legend as I felt it had more humorous elements than the subsequent follow-ups.