HomePod MultiRoom - Cannot get it work reliably

Hi all,

I love my HomePods, 2xOG's and 4x Mini's, however I simply cannot get multiroom audio (playing the same song in multiple rooms) to work reliably at all. The 2 OG's are in stereo pair acting as speakers to the main TV's Apple TV (which works fantastically). All running latest HomePod OS (16.1 at time of writing).

I have obviously the everywhere group and two other groups of upstairs and downstairs. But it just never works - it will usually just play on the HomePod it is requested from and then the others will either have nothing at all, patchy audio (might play for x seconds then cut out for y seconds, rinse, repeat) or sometimes some of them will work correctly.

Does anyone else have this, or is aware of a fix? My professional background is in network engineering, so I'm not adverse to implementing whatever the fix may be.

I have an Asus RT-AX88U router on BT FTTP. I have tried on stock firmware and the Merlin firmware. I have tried all sorts of suggestions found across the interwebs, including the stock settings, the recommended settings from Apple, the settings found on this link on Reddit. I have tried just about everything else inbetween.

I have also tried using the BT Smart Hub 2 provided by BT, just as a wifi AP. It seemed to make things better, but not really.

I moved over from Amazon Echo's and they just worked no issues. I appreciate the HomePods use AirPlay protocol which some routers don't play nice with, but I have tried so much at this stage, I am just lost as to what to do next. I'm not even against buying a new Wifi router, or mesh network, just as long as I can get it to work!

Please, can someone help me?! :)