Rear brake doesn’t grip/slips through and doesn’t stop the bike sharp or at all
This sounds like a classic case of pad contamination. You are well advised to try to determine the cause to avoid it happening again.
My first suspicion would be your washing process. How did you wash the bike? Is it possible you got a cleaning product on the pads?
I'd definitely inspect the calipers very carefully to make sure you do not have any leaks. Clean the calipers and rotors thoroughly with brake cleaner, then with the old pads or a brake block in place try squeezing the lever hard a few times. Recheck for any fluid leaks.
If you are getting good resistance when squeezing the levers and there is no 'sponginess', it's unlikely that bleeding is required. You said you bled the brakes more than five time, I do have to wonder whether you are maybe casing contamination when you are performing the bleeding.