Update: Just wanted to say thanks to the support from tp-link. Fitz Peng from the technical support provided with a beta firmware and solved most of the issues. I have since moved on to the new fimware: Archer BE800_V1_1.2.1 Build 20250108

I can report that VPN client works as described. However, do note that it appears that only ipv4 addresses are supported so I had to turn "off" ipv6 functionality to stop traffic from slipping through.

There is some oddities with DOH/DOT DNS that I will not elaborate here.

It appears that while 6Ghz speeds are faster than 5Ghz speeds, the MLO network still doesn't seem to be implemented correctly. I actually get 2.4Ghz speeds or lower if connected to the MLO network. Perhaps someone might have an idea on what is going on?

Overall, I believe with newer firmwares, the situation should improve. Well done tp-link support and technical development team.

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); game console, TV, plus two other devices <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 phones, 2 PCs, 1 Mac. <5 devices>

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

<5 Google Chromecast 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