If the Garmin forum was not helpful how is anyone else going to help.
You should of never used Windows to format the unit.

Then you talked about it running slow.
As you never said what was slow about it.

IF doing a Factory Reset and starting over did not help.
Try contacting Garmin to see if you can get it back in working order.

Maybe someone else wants to deal with your problems as I have run out of things you can try.