I have limited experience with this, I’ve been modelling some plush toys that are being printed into 6 inch figures, although the they were fairly simple in design/detail compared to what you’ve been tasked with, I think some things to consider are, wall thickness. There’s a minimum thickness that should be adhered to, this may very depending on which printer and material you’re using, if the model you’re printing isn’t just a single solid piece. making sure there’s no holes or gaps in the mesh. Everything needs to be air tight, as you’ve seen with your current model, fairly decent topology so the slicing software can do it’s job and sort it into layers etc.