Elaborate usability tests are a waste of resources. The best results come from testing no more than 5 users and running as many small tests as you can afford.
https://www.nngroup.com/articles/why-you-only-need-to-test-with-5-users/
* user testing across many small tests * cost-benefit analysis of user testing * Nielsen, Jakob, and Landauer, Thomas K.: "A mathematical model of the finding of usability problems,"