Hardware Version and Firmware Version updated.

Current network configuration:
10Gbps XGS-PON Bridge, connected to BE800 WAN port.
BE800 with a total of 20 - 25 wired and wireless devices.

Connected devices: (devices with static IP address are in bold)

Ethernet (Wired):
PC (10Gbps); PS5, Soundbar, TV, IKEA Tradfri base <5 devices>

Wireless: 

networkname - 2.4 & 5 Ghz:
6 devices + 2 to 3 other devices that connect from time to time. <up to 8 devices>

networkname_6 - 6Ghz: hidden and unused

networkname_7 - 7Ghz MLO:
2 android phones, 2 laptops, 1 Apple laptop. <5 devices>

networkname_IoT - IoT 2.4 & 5Ghz: (they share the same network name) 

2x Google Home Max; 1x Nest Audio; 1x Nest Hub Max; 1x JBL Link <5 devices> (these devices are also grouped together in Google Home)

Coverage area is around 60 square meters spreaded over 3 main rooms, BE800 is in a relatively central location. I did a network map scan with WiFiman and the furthest point (where there are no devices) within the house still has around -60dBm of signal for all

Problems:

(1) Unable to cast from Tidal, if it plays, I have stuttering from one or a few speakers if not all. Casting from Spotify to group in Google Home appears to be OK (each device appears to show around 0.3-0.5Mbps of downstream traffic). Tidal seems to take at least 0.9Mbps of downstream traffice per device.
I have been battling random disconnection from various Google Home devices. They seem to take turns to go on vacation. The situation appears to improve for a few days whenever I try a new configuration then begins to deteriorate again. The current configuration (above) keeps them connected, with internet, most of the time but I am unable to cast from Tidal. Casting to a single google audio device is OK. 

The initial setup had all devices, except those listed above on the 7Ghz MLO, on the primary 2.4 & 5Ghz "smart network". With this initial setup, google audio devices would randomly lose internet connectivity without any apparent reason over time. Only a restart of the device, and/or restart of disconnected device and router appears to resolve the issue for a few hours / days. This initial setup seems to allow me to stream from Tidal. But I haven't fallen back to this setup yet, as it was easier not to have to deal with disconnected devices.

(2) one specific phone or laptop would be connected but without internet on networkname_7

This seems to come and go. But I ruled out signal strength as this is basically user sitting right across the router on the sofa.

Could someone kindly point me to a specific setting to tweak or maybe shed some light on how to read the logs? I think I could try to solve this on my own if this is a user-problem.

I am assuming that this is not a server-side (ISP) issue since all wired devices behave normally: I consistently get 5-6 Gbps download speed on the PC.

Thank you in advance for any help. I will check hardware and firmware version later today.

linking to other post that are possibly related:
@mkostersitz link to post
@motorider link to post
@attawut link to post