Apologies for remaining silent for the past week - I had ordered stuff and wanted to wait how things turn out. You are right - your description is very close to my setup. It’s probably not logical but I wanted to continue with the mesh software to see how things work compared to the Asus setup and to the initial try with the Omnia and cheap dual-band mesh-nodes.
I received the 7916-NPD and installed it in the left slot. I originally wanted to go for the right slot but the sockets were in installed to match the short card (are they soldered?). The 7915 pigtails remain connected to the same antennas as before. I kept the diplexers (2.4GHz connectors unused) as I ordered three new antenna sockets only. The NPD is connected to three new antennas sitting in the pre-drilled holes on the back. I had to install the kmod-mt7916-firmware package to get the new card recognized:
02:00.0 Unclassified device : MEDIATEK Corp. MT7915E 802.11ax PCI Express Wireless Network Adapter
03:00.0 Network controller: MEDIATEK Corp. Device 7906
[ 9.881807] mt7915e 0000:02:00.0: HW/SW Version: 0x8a108a10, Build Time: 20211222184017a
[ 9.920769] mt7915e 0000:02:00.0: WM Firmware Version: ____000000, Build Time: 20211222184052
[ 9.951895] mt7915e 0000:02:00.0: WA Firmware Version: DEV_000000, Build Time: 20211222184111
[ 10.208149] mt7915e 0000:03:00.0: HW/SW Version: 0x8a108a10, Build Time: 20211230135620a
[ 10.270890] mt7915e 0000:03:00.0: WM Firmware Version: ____000000, Build Time: 20211230135720
[ 10.321480] mt7915e 0000:03:00.0: WA Firmware Version: DEV_000000, Build Time: 20211230135808
The backhaul is running on the 7915, AP for 2.4 and 5GHz clients on the 7916.
Because I have already spent too much money I wanted to first check how a tri-band router supported by OpenWRT performs, so I got a Linksys MR8300 which has an Atheros QCA9886 for 5GHz channels >100 and an Atheros IPQ4019 for 2.4GHz and the lower range of 5GHz channels. I chose 5G ch 48 for the backhaul and 104 for client AP.
The problem is that the connection is incredibly slow via the mesh node. Whilst indicated client <> mesh-node connection speed is about 860MBit/s and mesh node <> Omnia about 650MBit/s the datarate client <> Omnia (iperf) or WAN is a fraction (80kB/s). It gets better for larger downloads but smaller transfers. e.g. reloading a grafana dashboard on a Raspberry on LAN or accessing files on a samba share are unacceptable.
In order to rule out any HW issues: is it OK to install the 7916-NPD as described? Is it OK it is listed as as “Generic MAC80211” in the wireless overview?
Edit - upon further inspection I believe the MR8300 is the issue. I will set it up again tomorrow. However, the question remain, wether the 7916 is recognized as expected or if I need to upgrade something. Turris OS 6.4.1 (kernel 5.15.120).