I'd certainly stick with the Zyxel to find out. Unfortunately that's a minimum of 10 days stability, with a probable maximum of 14. Also jot down the error counts once per day at about the same time. Just for long-term comparison if you put the HH back or make any other change later.
I suggest you make sure you can identify (and avoid) that filter and cable. Possibly try them (one at a time) a fair while after/if you get the SNRM drop.
You didn't say anything about possible new noise sources, in my previous post?
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