A4 Avant: Cracked Rear Brake Discs

Dear fellow enthusiasts,

While I was changing the tires on the car, I noticed that both brake discs on the rear axle have many small, radial cracks. The discs are still thick enough and have approximately 144,000 kilometers on them.

Despite regular maintenance and inspections of my car, I must admit that the cracks probably didn't appear just last week .

The new brake discs and pads have already been ordered and will, of course, be installed immediately.

Despite this, I would simply like to ask the experts here how (safety-) critical these cracks should be assessed.

I still have a slight vibration in my braking system (the front rotors are practically new, the old ones were warped). Could this be related to the cracks?
When braking using the handbrake (only with the handbrake!), I cannot detect any vibrations. But the handbrake is not hydraulically connected to the brake pedal .

At the same time, I would advise all A4 and (G3??) owners to carefully inspect their rear brake discs.