It seems unlikely that you’ll resolve this in the space of a day, given that you’ve probably eliminated most of the obvious network issues by moving to the Netgear Orbi (or there’s something very odd happening which means they’ve propagated across). If this were me, and this were my last chance to return, I’d seriously consider doing so. If the fault indeed lies with the Bluesound/Roon interaction then past experience has shown that this could take quite a while to address, if at all, and you may not wish to chance that. The frustration is there isn’t anything else out there quite like the Flex (RAAT, same form factor, same price).
Edit: OOI does it still happen if you group the speakers in the Bluesound App but then play via Roon?
Edit 2: It’s worth checking out this which suggests that the very recent BlueOS update would be worth at least trying (though it’s a slightly different issue).