HTTP 500 error on publish only after adding one model

before the halving, try this: export the model, open a brand new baseplate, drop it in and publish that. if the new place publishes fine, it's something in the original place file and not the model itself, and you've saved yourself the search.

Split the model in half, delete one half, publish. Whichever half still breaks it, split again. Five or six rounds and you've usually got it down to one part or one script. The things I'd suspect first with "a few scripts and a lot of parts": a union or mesh that never uploaded properly, a script holding a huge string (people paste whole data tables into them), or a package link inside the model. Also try publishing the model on its own into a blank place, if that fails too you know it's the model and not a size thing with your game.