Router with monitor and promiscuous mode

Hardware Questions and Recommendations
2

Qualcomm Atheros wireless chips generally support monitor mode. For example:

jeff@office:~$ iw phy | head -n25
Wiphy phy2
	max # scan SSIDs: 16
	max scan IEs length: 199 bytes
	max # sched scan SSIDs: 0
	max # match sets: 0
	max # scan plans: 1
	max scan plan interval: -1
	max scan plan iterations: 0
	Retry short limit: 7
	Retry long limit: 4
	Coverage class: 0 (up to 0m)
	Device supports AP-side u-APSD.
	Available Antennas: TX 0x3 RX 0x3
	Configured Antennas: TX 0x3 RX 0x3
	Supported interface modes:
		 * IBSS
		 * managed
		 * AP
		 * AP/VLAN
		 * monitor
		 * mesh point
		 * P2P-client
		 * P2P-GO
		 * P2P-device
	Band 2:

I haven't worked with any MTK, Marvell devices, nor any Broadcom devices in recent history. Broadcom wireless has very weak open-source support, in most cases.

See also https://wireless.wiki.kernel.org/en/developers/documentation/mac80211 and related