Following up. I downloaded and installed the latest stable of BetterDisplay (formerly BetterDummy), and also moved away from DisplayPort on the dock. I'm now coming out of HDMI on the MacBook directly, out of HDMI on the dock and to HDMI IN on both monitors. What that gets me to is 3840x2160 on both monitors, with the Main display at 60Hz and the Extended display at 30Hz. Can't get a better refresh rate on the second display, but I guess I'm going to have to live with that, UNLESS someone can tell me that there is another dock out there that would be able to deliver 60Hz 4K on the second monitor.
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