Is your org tracking time in WF? Of course that's the best metric for capturing time/effort, you could pull hours from all of the sponsorship-related projects. If not, then consider looking at # of projects per sponsorship I guess (which isn't great bc you could have 1 large project that takes more time 5 smaller projects).
You could have each sponsorship be a Program, and put all the various projects/deliverables under the respective program to easily see # of projects per Program (sponsorship). Consider creating more templates to cover the types of work you do for the sponsorships. Then whatever the sponsorship requires, you have templates ready to go or to at least use as a good starting point to tweak since it sounds like your 2 general templates aren't providing much value.
If this helped you, please mark correct to help others : )