What’s the best way to set up SUM or UX KPIs for a long-term project?

I'm working on a long-term expert level industrial software as a lead UX designer. We would like to implement and measure a set of key performance indicators (KPIs) for overall UX quality or SUM.

What's the best way to go about this? Can you give me an example of how to determine what methodologies and KPIs should be measured over time? Thanks!