I also had this issue and the solution I found was from Blupry. I bought this template https://www.blupry.com/app-details?recordId=rec3BuCx4b2ouoZEP and it works perfectly without any issues. Hope this helps you as well!
https://community.flutterflow.io/ask-the-community/post/how-do-i-add-clustering-to-mapmarkers-on-google-maps-cWcqhdWZVMjTP21?highlight=TnsCKGRLSHBocvb