Fitbit support incredibly unhelpful
Awhile ago I tried to do the Google migration back when they very first started pushing it. It failed and then wouldn't let me. Now here we are approaching the deadline so at the start of the week I tried again. Didn't work, says my Google account already has Fitbit.
I spent a few hours trying all the solutions I could, went through my Google Account and deleted everything I could find with Fitbit in the account, then waited a few days and tried again. It failed again.
This time I reached out to Fitbit support who told me my primary account doesn't exist, but the Google one does, so my account successfully transferred and to wait a couple days for the data to move over.
I told them no, it didn't, it failed every time, and then gave them the account ID. Suddenly they found my account and it hadn't successfully transferred.
I asked them if they could delete the Google Fitbit account in order to transfer, they told me it takes 90 days to do that so they won't be able to do that and I need to make a new Google account.
To me it sounds like the initial migration failed in some way, half linked my Fitbit account (seriously there is no longer any sign of Fitbit data anywhere in my Google account) and they can't/won't fix it.