Google Certified Tech for Account Security

I need a Google Certified Tech to help me clear up my accounts with Google as a webmaster who I hired, without prior authorization and against my wishes, granted himself SDK and Cloud IAM Creator access to my Firebase account and possibly mirrored my laptop through Google Meet's screen sharing function. I have a Google for Nonprofits account which once approved will give me access to most of the resources Google provides, but I am uncomfortable with the transfer of my domain and website to the Firebase account as this individual would have full access potentially to my financial data and Google Ads. Although I followed Google instructions to remove access to this individual, I noted his email address is still connected to my IAM account. My first desire is for my current website and domain to be transferred to a Google Website. I'd like to use their platform and I'd like to make certain there isn't a "back door" access granting permissions to this rogue webmaster.