It seems when we think about usability testing, we think we need to conduct a separate test cycle to focus solely on usability issues. And perhaps we do (need to design distinct test sessions) because there are only so many things we can think about and focus on at any one time. But I think it may be feasible to think about usability testing as we execute functional testing eliminating the need to gain an additional test cycle. It also seems feasible to focus on usability during compatibility test cycles such as when we're executing a subset of tests to test on different operating systems or different browsers or different mobile devices.
Depending on the types of usability you're focused on, perhaps a short checklist of ideas could help testers focus on checking usability as they test.
I don't rely on metrics to report status to people or to provide a sense of coverage. Instead I find non-numeric ways to debrief people about status. Storytelling is one of the ways I communicate with people. In the situation you've described, I might be inclined to outline user personas as a way to discuss what usability items to test and then use those same personas as a means of communicating back with the design team.
One additional comment, you mentioned that the design team you work with is not good at collaborating. I'd suggest that testing of usability might be a great "bridge" topic that you can use to help build better communication with the designers. As your test team identifies issues, you might look to work with the designers ahead of time so that issues can be found even earlier in the process and you can continue to build better communication across the teams. Working the best you can with each team is one way to improve a product. Maybe you can get the designers to be better at collaborating with you if you find ways to work with them. I believe another SearchSoftwareQuality expert, Mike Kelly also responded to a similar inquiry. Here is Kelly's response to the question you posed, How to test usability for a positive user experience
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