About 9 months ago we had an interesting exchange on cleaning cloth wires. Here is the link. https://pinside.com/pinball/forum/topic/how-do-you-clean-cloth-covered-wire-harnesses
Please review the video in post 6 its a hoot plus what some of our friends do. This is real and it is safe ....if done overseas anyway! I do not do this however as the magic cleaner I think is like our Drano.
I do however take the harness of like you (leave relays on) and soak a couple of hours in a degreaser, simple green and water, then brush the parts, rinse and force the water out with compressed air and leave for a week to dry. Big improvement and no issues yet.
Steve J