Reinstalling Windows 10 on mid2011 iMac with Bootcamp
I’m assuming you have a bootable Windows 10 pen drive with Mac drivers.

I didn’t read any tutorials for doing it, the following is what I would try given your circumstances (and considering I’ve already installed Windows a few times on a few different Macs for gaming).

I would try to go on with the Bootcamp Assistant even though it says Windows 8, follow everything to the end, and ignore all the steps you don’t need, just so it creates the partition you need.

Windows 8 drivers could work on Windows 10, but if you already have drivers, ignore it too (leave it unchecked). Do not insert or let the assistant erase or do anything to your bootable pendrive.

Then when the Mac reboots, insert your pen drive with the Windows installation. If your Mac doesn’t automatically boot the pen drive, reboot again holding the option key and select the pen drive.

Install Windows normally on the partition created by the Bootcamp Assistant, and then install all drivers manually in case it doesn’t automatically do so.