You can set and cover the steel plates and either GPS or triangulate the location. You can use the Pipe guide to set a platic 4" pipe to some safe distance below your milling depth. You could use the Newcon 12" auger to quickly cut down and expose the pipe. You will most likely break the pipe in this process but the pipe is very reasonable to replace. This will give clear access to the Key shaped hole in the steel plate for quick cutting and pulling. Alternatively you could simply bury the plate and use the Newcon 12" auger to cut all the way to the plate and expose the Key shaped hole for cutting and removal.

Resource Link: https://mrmanhole.com/product/cut-pull-and-plate-package/
Additional Link: https://www.youtube.com/watch?v=Hpl0VvpwFS8