Specific Flange Repair Request

Concrete floor, 4” ABS with rusted steel flange.

ISSUES:
1. Top of flange is about 3/16” below tile.
2. Flange is rusted bad enough that it will not hold bolts much longer.
3. Concrete is not poured up to ABS. Nothing to screw into.
4. Rear tile is about 1/8” higher than front tile. Not sure why.

INITIAL OPTIONS:
My plumber suggested a replacement flange that twists into the 4”. This would
need to be anchored with screws into concrete, correct?

Possibly use the Moss Bay flange?
This is from another thread.
“if you want to get yourself into tons of
trouble go ahead and cut that ring off that
plastic flange embedded in concrete.....”
It also does not solve the height issue and need to be anchored.
How can you be certain that it will fit?

Use a repair plate and just screw it into the existing flange?

Recommendations Please.

Also,
When shimming, do you place the shims under the ribs running across the toilet or under
The edges?

Thanks again for the help.

I shim from the back, pinning the front of the bowl down.

Thanks,
I have removed the rusted ring and sanded off the 1/4" lip on the inside of the 4". A new ABS flange with a SS ring slides nicely into the 4" solving both the height and rusted ring problems.

There is about a 2" gap all the way around the ABS that extends to about 8"deep so there is no place to anchor the ring. I have noticed that this is very common here. They just let the plumbing hold the toilet. Surprised it ever met code.

What is the correct way to finish this? I was thinking of sand backfill topped with 3" of thinset around the pipe to hold screw anchors.

Thank You