The doctor trigger is indeed a thing. When my MIL was showing enough signs of dementia, while being caregiver for FIL (wheelchair bound and dementia), Adult Protective Services got involved. (After she tried to pull FIL out of the hospital when he clearly needed medical attention.) It was a long process and MIL's doctor tried to refuse to issue the statement confirming her dementia till APS pointed out she was legally required to fill out the paperwork (this was Kentucky, btw). That reluctance dragged the process on 2 extra months.
Our trusts call for either doctor and spouse agreeing on incompetence or 3 people from a list. In my case it's DH, my sister, and my BFF. In my husband's case it's me, my sis, and his sis. (We are close to my sister).
The estate attorneys mentioned the reluctance of doctors to state competency - hence the 3 people in agreement as a backup.