I want to place the router optimally, but see no info on antenna properties. Are those internal antennas considered to be “omni-directional”, or do they have a certain directional pattern?
It doesn’t matter, the antennas are placed so that unit can be oriented in any direction.
Also, the antennas are quite small, so it doesn’t really matter.
aight thanks. Will still test though 
With the short wavelengths, moving the router a few centimeters in any direction can make a significant difference in signal to a particular device. When we mesh a couple of hAP ac-lite units on 5GHz, we can see 10+dB difference in signal level when I move one device the width of the device. That’s because of the obstructions in the house, not because the basically ball pattern of the antennas has shifted.
You can look at generic diagrams for 2 dBi antennas. Physics are physics.
With antennas so tiny, it’s not possible to make an accurate radiation pattern. Not that it would help much in planning or orienting the device. Better to adjust on site