Latest seems to be that defo moving core to iMac rather than QNAP has drastically reduced the drop outs.
Been running on 3 Meridian Zones (2 running at once) and things seem to be broadly stable. Occasionally my iMac with local core will freeze up, but that's more an iMac freezing when full (everything in my house runs on it).
iMac is full SSD and top end so possibly enough processing.

So, putting SSD into QNAP may be an option
iMac continue (compromise known of running on main computer)
Put in dedicated (NUC / Nucleus etc.)

Good learning..... more to come.....

M Customer since 1991...
2 Channel: 818v3 into 8000SE's running Roon on a Nucleus One with QNAP TVS-871-i7-16G 8 Bay NAS
Cinema: 861v8, 2 x M6's, 7200HCSE + DSW
Office: Explorer 2 into Shure SE846's
Kitchen: AD88
Other Stuff: Philips 77" 4k OLED Ambilight TV. Hue Lighting System, ATV 4K & a lot of Apple stuff...