You could aim for a specific conversation rating via the customer satisfaction report

Ya I tried that. There are 2 challenges with that-

1. it might seem impersonal
2. most of the customers opt not to rate

So it doesn’t seem to give the right picture of how our support is doing quality wise.

inly other way I could think of is auditing the conversations. But it is a lot of work.

Any way I can better understand how my support reps are doing?

If you want to focus on the quality of support I don't think there is much more you can do besides these two options. At least I'm not aware of anything else. Also not sure if auditing the conversation will give you a good metric (since it's very subjective). You might want to do some research on the topic of OKRs for support teams, there's some good reading on the net

Further to @kevin b​'s reply, have you check out our Ultimate Guide to Conversational Support, @sarita​? It's our most recent book that we've published, and it goes into support structures and KPIs in good detail!