Can you give us your opinion?

Hi all,
I am currently validating a business idea related to UX/UI for e-commerce.

If you are a UX/UI designer, e-commerce Product Owner/Manager, e-commerce Analysts (etc.), it would be great if you could answer this form UX creation - E-commerce (it will take you less than 10 mns) OR share your feedback about these points:

a) what are the KPIs you use to measure the UX/UI performance on your e-commerce?
b) what are the biggest pain points you perceive during the process of measurement of your UX/UI choices?
c) Could you please please please tell me WHY?

Any other input would be amazing!