I'd like to measure usability and identify potential areas for UX improvements of rather big application. We've managed to identify 3 main user groups (based on roles) and around 3-4 main tasks performed by each of these groups. The app has around 200-300 users in total.
1st thing I'd like to do is to send a questionnaire to all users- but I don't like the idea of asking about the overall usability (e.g. with System Usability Scale (SUS) questionnaire).
I'd like to measure usability of each of the tasks for each of the roles so that when results come I know that for instance role 1 views task 2 as hard to perform or unusable, but I want to avoid asking same questions about each of the tasks (sending a SUS for each of the tasks). How should I approach this? Is it acceptable to send one for each of the tasks? How would you perform such project?