Stuck in endless macOS Sierra installation loop
This is the type of UX pain that really irks me, and very unlike Apple.

Since you're a long time Mac user I'm going to assume you have access to another Mac, so my suggestion would be to try reinstalling a fresh version of macOS Sierra via a USB installer.

Follow the steps below:


On another Mac, download the macOS Sierra installer via the Mac App Store 
After its downloaded, the installer will launch automatically. When it does, quit the installer.
Grab a USB flash drive of at least 8GB in size
Connect the USB flash drive to the same Mac
Rename the flash drive to Untitled
Launch the Terminal app 
Now enter the following command into Terminal. It may be best to just copy it from here (try triple-clicking) and paste it in. 

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app
You will be prompted to enter your user password. Enter it now and press return (note that you won’t see the cursor move or display any of the characters you enter)
At the next prompt (about erasing your drive) press the Y key
Wait for your USB flash drive to be erased (you’ll see the progress via % values appearing within the Terminal window)
Once the erasure is complete, you will see a prompt that files are being copied to the disk. This may take a while, so be patient.
Eventually the copy process will be over and you should see a Done message in Terminal. You can now quit Terminal.
Eject the USB flash drive and now connect it to your iMac (the one you want to reinstall macOS Sierra on)
Startup the iMac holding down the option key
Select the USB flash drive you created from the list of drives and press enter


You should now be able to boot from the USB drive and install macOS Sierra from there.

Let me know if you have any questions.