Since you dislike the Mamiya strap, why the reservations about simply cutting it up, stealing the lug connectors, and throwing it into the trash?
The RZ works fine for me on the stock Mamiya strap. I attach the lug connectors on the strap so that the lens points upward for over-the-shoulder carrying, and the WLF rests against my gut for around-the-neck carrying.) I would hate to carry it, or any heavy camera, on a neoprene strap; especially over the shoulder. I'd go with the Up-Strap or another fixed (i.e. non bouncy) strap, since you dislike the Mamiya strap. I can only stand the neoprene straps on lightweight cameras, personally, and even then they still feel a bit sleazy to me.
FWIW, this (http://www.bhphotovideo.com/c/produc...k_Release.html) is the strap I use for my Mamiya C series and Press series cameras, and my near-7-lb. Linhof Technika III. With so many cameras, the quick release is an extremely beneficial feature, and it is simple, elegant, comfortable, and old fashioned, yet with all the modern features that really matter in a practical sense to me (nylon strapping as opposed to cotton, and quick releases), without being some high-tech spandex superhero strap. Also, you can easily remove the manufacturer's name tag to make it more plain, which I like. The suede never slips; ever, and I'll wait till my brick of Linhof or my Super 23 gets ruined in a fall to agree with all the people who swear that using a quick release is like asking for camera suicide. The strap is perfectly solid. (You want to feel SCARY? Put a heavy camera like my Technika on an Op-Tech strap and feel it go bouncy bouncy as you walk!) It is also quite affordable. This simple suede strap is perfect for me, and I am glad that it is made.
As you see, it is entirely a matter of opinion, and you already have yours, so I'd ignore all the noise and just read the possible answers to your question that have been provided. E.g. Chop that Mamiya strap to pieces and call it a day!