I have a 6 month old Macbook pro and I also have a 2014 Mac -- I tried both with USB DAC and they both had dropouts. I have the buffering at 1000 and have fiddled with everything seemingly related to no avail. I'm not sure I can blame it on Time Machine........ Right now - I have TM off and will leave it off for a day or two as a test. I didn't check if the MacBook was backing up when I got the dropout, but 2018 MACs have other audio problems so I haven't repeated. But also, when I used the laptop, the library was still with the 2014 desktop Mac - so not a good test.
As a last ditch effort - I just bought a Raspberry Pi 4 (I have a Master license) and will attempt to use it as a client with the Mac as the server. I have the possibility of either USB/DAC output or HDMI with the Raspi. If the Desktop Mac controlling the library still is the problem, I can set up an external USB drive with the Raspi, I also have a 8 year old Windows PC (mini Dell) in a box somewhere - I could dedicate it to MC.
But, for me, the MAC is the best solution (different computers will complicate my world) if I can get it to reliably playback (but all in all, I've tired of serious troubleshooting with this one). This seems a problem worth escalating to Apple tech support if TM related. Does JRiver have a contact at Apple for this sort of problem?