[EC900]

Since you’re aware that rear wells tend to collect more oil than fronts, I agree with use of Simple Green or Dawn Heavy Duty cleaners and work well, even tried bbq grille cleaners to break down oil/grease (not very effective). Gunk is messy, smelly and has its own residue to deal with.
Spray SimpleGreen or Dawn on and let it soak a bit, the. work out the dirt in the crevices with a stiff paint brush (cut the bristles down). You’ll probably need to wash/rinse/repeat a couple-few times. Be sure to blow out with compressed air to decrease dry time to inspect how cleaning is going. Power wash after soaking can make quick work - but too close will lift off previously applied aftermarket undercoating.

Over-spraying with fresh Wurth may just “bridge” that old undercoating and bonding might not be adequate even if thoroughly cleaned. Old stuff too brittle?
You may want to try a wipe down with toulene beforehand (Note:Very volatile, open space and mask respirator needed) to pretreat the old undercoating surface a bit may help, really depends on its condition and results will vary. I’ve used denatured alcohol as a prep, and Odorless Mineral spirits may soften up the old undercoating for prep too, evaporates slower but will do good on removing oils too.

My experience: When I got my 911, the wheel wells were caked with decades of dirt, mud, lots of oil saturated mud and the oil film. Looked sloppy and neglected against shiny ArmorAll-ed tires.
I soaked wheel wells with above cleaners, let it set in to soften the dirts, then power washed (rinse) using wide fan spray nozzle and kept a reasonable distance just enough to get the dirt out. Once the bulk of cleaning was done I worked on the details and hard to reach areas. This is tough on jack stands, laying down and splashing water all over. Using a small hand pump sprayer (vs. trigger sprayers) will be useful to dispense cleaners at odd angles. A larger 2 gal. garden sprayer was used to rinse, less splashing.
Oh…and wear goggles !
Since I didn’t have any additional black undercoat ever being used, my original painted undercoat for the most part was perfectly preserved. Not bad for a car that never has wheel wells properly cleaned other than maybe car wash and rain water over 48yrs/200k.
So yes, your original is probably in good shape also if gravel roads weren’t beating them up.