Appraisal incoming: how to find out your design impact?

I've been in UX for the past year and have my first appraisal coming up. In that time I have worked on a multitude of website and app designs and redesigns. I have received many positive comments along the way from both colleagues and clients. I am the only designer in this small agency.

When I have tried to talk about measurable metrics among my colleagues (like improved bounce and conversion rates etc) that line up with the client's goals, I have been told that it's unlikely to find out the real impact a design has had because of other areas like improved SEO and marketing that can skew the results.

There is no current process or focus in the company on performance metrics.

My appraisal is coming up, and I want to advance in my career. I know it sounds naive, but how can I find out the impact of my designs on the client and company as a whole?