Sara, we're using the Resource Planning tool for this for the same
reasons you outlined in your idea description. However, we're not able
to see unassigned hours by project or job role like you can in the
Scheduling tool. I added an idea last week around this. If you think
it's helpful for your case please up vote!
https://support.workfront.com/hc/en-us/community/posts/360014264814-Planning-Tool