I've been trying to get Google to merge my organisational GMB with a departed practitioner's GMB for about 6 months. Google has explicitly told me this cannot happen as the practitioner "owns" their GMB, not the business.
As I'm the GMB admin for both profiles, Google suggested marking the practitioner's GMB as 'perm closed' - the obvious problem being that when the practitioner's GMB shows up in search, people are likely to take this as my business is perm closed.
I've simply decided to remove most content (images, posts, website etc) from the old practitioners GMB, not 'review updates' and essentially let this GMB profile decay so Google doesn't serve it up in search. Fingers crossed...