I’ll just drop a link to this discussion on integrating two or more projects together. To be clear this is a PC/Mac workflow, but to me that does not sound like a problem since you essentially do not want your research to sync anyway.
As for getting to that point, it is very easy to split projects. You basically use Save As to create your research project, then nuke the Draft folder contents or anything else you don’t need, then do the same in inverse for the writing project that gets synced. You aren’t copying or rebuilding data if you start with two identical projects that only have the stuff removed that you don’t want from each. You can have 10,000 items or 1,000, the amount of work is about the same.
Naturally, when embarking on such a task, making sure you have a good solid backup of the original unified project is important. Use File ▸ Back Up ▸ Back Up To... to set aside a copy in a safe place.