The new code that splits posts according to the character limit on a target instance works with the latest update of the #BigBone / #Yuki Java library on which #MastodonContentMover depends (thank you @andregasser and @bocops) and is pulling the instance limits correctly from the instance. That was the last major test this update had to pass.
I'll keep testing this version with real data for the next day or two then hopefully release it before the weekend.