I’ll try and answer this as best as I can. I’m sure many will correct me?
1. The BSTheatre has 7 speakers and as a standalone soundbar, it can fire those speakers to make a pleasant go of spacial sound. I don’t think it can compete with a multi-speaker setup of its own tuning.
2. The BSTH has 4 wired Powelink paired connections and I believe 4 WISA paired connections. For a 7.1.4 system, you need 5 pairs of speaker, plus, possibly a subwoofer and the Theatre. This will mean some aspect of your setup will need both wire and wisa.
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3. If all 7.1.4 channels are presented to the HDMI input from the source and processed through the Atmos, but there are speaker groups missing, the BSTH will down-sample to the best output.
4. I believe when external Beolabs are added, the side and up-firing BSTH drivers are disabled. Why that is I don’t know just yet but that is what I observe in my system.
I personally feel a traditional 5.1 (2 fronts, centre, 2 flank) loudspeaker set is enough sound output without occupying too much real estate and clutter in the room that a further 3 sets would entail.