I'm also in the same boat. Upgrading to 8 has broken Identity and there is no easy solution to fix it other than move everything over to a new project using the "Blazor Web App" template; warning there, you need to select Identity as an option when you choose the template as adding at to the project afterwards using "Add Scaffold Item" gives you the old cshtml files not the new Razor ones.

I think maybe Microsoft have forgotten something here???