Are you the first owner of your Zenith? Mine was purchased in 2016 and is unfired, but I am thinking about firing it. It's so hard to look at and not shoot beacuse it is my only "MP5". Reason I ask is, since mine was one of the first preorders I am wondering if the issues you addressed above were more prominent in the earlier Zenith's.
I have two RS's and a K model, both from the 16 batch, and have had zero issues with any of them after thousands of rounds. I think the late 18 or early 19 batch is when the issues started.
Megabyte I am glad to hear that they made it right! I second everything you said, their CS is fantastic.
My understanding is the bad “batch” was the first few months of 2019. Not to say there couldn’t be a few bad ones from before but the batch people typically refer to as having excessive mag movement and bad finishes, seems to be mostly from early 2019. I played with Z5 RS a local shop just got in and the mag was as tight as could be. And the finish looked very good to me. Not quite Sp5k good. But certainly not bad at all.