You should create separate SSIDs for 6GHz and 5GHz, you want to stick to one or the other for the WiFi link. When using multi-link, the dongle will switch to whichever is not being used by the WiFi connection (Or always 6GHz, if the option to reserve it for the dongle is enabled). You are not using two 6GHz connections, it’s always 5+6GHz. (Maybe that’s why you incorrectly assume 3 frames is the maximum in a room?)

Your router connection being a tad worse is probably because the frames are using 5GHz there (assuming the dongles are on 6GHz), and doing so on the same router? You’ll probably get better results disabling multi-link, and using only the dongles.

Transmit power is not the final say on the router side, check for features like “airtime fairness” and “transmission bursting” that trade latency for other things.

You can absolutely have more than 3 frames in a room, though for that kind of setup, you’d want to configure each of them to use specific, non-overlapping 6GHz channels (which is tricky as the dongles use hidden SSIDs that can’t see each other, and therefore avoid channel overlap). And you would need to use the dongle for this, as otherwise you’d need to get one router for each Frame, to enable them being on different channels.

You’d also want to disable multi-link, as again, to use it effectively you’d need both a dedicated router for each headset, as well as a dongle, in order for all connections to use separate channels.

But, this means that at 160MHz channel width, you can have up to 7 Frames in the same space, (and technically also give each a 5GHz multi-link connection) each using an entirely separate and dedicated 6GHz connection. You might even be able to drop to 80MHz and do 14 Frames. The bandwidth should be sufficient.

Though when this saturated, you’d have so many people and their devices around, that people’s phones and PCs scanning for WiFi will cause interference. It might becomes necessary for people to put their devices in airplane mode to stop them sending hotspot queries in the 6GHz range.

Vive trackers operate at much smaller datarates, and use the 2.4GHz range, they should not interfere with Frame streams.