Sounds like you have a worn-out B3 (reverse) clutch set, if not others. This causes the metal shavings. The B3 clutches are also affected by forward motion and gradually wear down. if you do some searching on this and other forums relating to B3 clutches on 722.3s, you'll quickly learn more about the issue.

Likely time for a rebuild. You may have bought yourself a little, but not much, time with the fluid. You may want to source a good used unit as the parts and labor for a good quality rebuild will run you $2,000-2,500 to have a reputable shop do the work. MB spec's 9+ hours for a shop to remove/install the transmission from the car. The parts cost for seals, clutches etc. is a small fraction of the overall rebuild cost, even for factory parts.

I'm facing the same issue right now with my E500 (variant of same transmission) although I've not had any of your problems yet. I'm going to proactively pull and rebuild/re-seal my own transmission this coming spring.

Cheers,
Gerry