I guess some of this must be done in person with an agent, even if you are NEXUS approved.
Nope. If you are a NEXUS member in good standing, then you have already met (and actually exceeded*) the qualifications for GlobalEntry membership. So, as long as your NEXUS membership is still valid, just log back into GOES and apply for GlobalEntry, and you should get automatic approval with no need for an interview or further biometrics.
*NEXUS requires you to pass background checks and interviews conducted by both CBP and CBSA. GlobalEntry requires the same, but only for CBP. So, by being a NEXUS member in good standing, you have already satisfied GlobalEntry eligibility requirements. Hence, I think, why there is no further fee or interview required.