At least it is supported on the deprecated ar71xx target...
Deprecated means that 19.07 will be the latest stable release for this device as long as anyone port this device to the device tree based ath79 target.

Apparently, 5 GHz wireless hardware in this device is QCA9887, which is driven by ath10k, and it requires non-free firmware, so basically we shouldn't count on 5 GHz even if everything else works. Is my understanding correct?

Probably you only need to install the required driver/firmware packages like:
kmod-ath10k-ct-smallbuffers ath10k-firmware-qca988x-ct ath10k-firmware-qca9887-ct