If it’s possible to use Ethernet with any of the devices involved, it will probably improve your experience. Chromecast grouping can be pretty hard on even the best of WiFi networks; one of the devices in the group receives the audio stream from Roon and then rebroadcasts the audio to the other devices in the group over WiFi. If your MBP is also on WiFi, the amount of wireless bandwidth being used to stream to a 4-device group is roughly four times what it takes to stream to a single Chromecast device; the data is sent from your MBP to one of the devices in the group, and then that device sends it again to the other 3 devices in the group. Also, none of these streams goes directly from device to device; the sender sends it to a WiFi access point and then the AP sends it out to the receiver. This means that sending audio to a 4-device group takes at least 8 times the WiFi bandwidth of the audio stream if all of the devices involved are on WiFi.
I would suggest that you switch your MBP over to Ethernet; if that doesn’t completely fix the problem, the Chromecast Audio supports the Ethernet Adapter for Chromecast. Onkyo states that the VC-GX30 supports “Wired LAN Input via Micro USB”, but I’m not sure exactly what that means in terms of adapters that do and do not work.