I didn't feel too well this afternoon so decided not to tackle this again. Will try later in the week when I get some time.
No black bits in the fluid noticed during the bleed. The lever feels fine, certainly moved fine while the system was drained of fluid prior to refilling. I've got a seal kit, I'm fact I've got a few new pistons so may resort to they. Pretty sure I did at least one if not nothing of the every when I originally got them but I've got a lot of these brakes and some a few so memory is a bit hazy on the topic. Certainly a few sets in boxes which I've fully serviced the levers.
It's just bugging me. I had a magura mt6 front that I couldn't get a good bleed on but eventually sorted it. Rear was one and done. Weird.