OK, in my mind, as soon as you mentioned your proposed room layout, and particularly the fact you don’t think you can accommodate a centre channel, this choice became a lot clearer. The centre channel is of such importance in a surround setup - some say, and I agree, the most important channel. Without it, I personally wouldn’t bother with multichannel. Nearly all the benefits I described in my previous post disappear if you can’t use a centre (and a full-size matching one at that). If it were me, knowing how critical this is, I wouldn’t waste money and room space on more speakers, I’d just go with 2x 8KSE/818v3 and be really careful about room/screen/speaker placement.
If there’s a way to accommodate a centre, though, then all bets are back on. With an acoustically transparent screen, you could be uniquely suited to a 3x 8K fronts setup, which not many folks can accommodate. But I guess to do so you’d have to explore using another part of the room other than the fireplace - I’m not sure how fire resistant Meridium is!
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The room is nice, but has an existing fire. We could look to reposition or remove but I am not sure I would get that one past the executive committee

The only real option for a centre is an in-wall speaker. Again not ideal but possible, but it will be quiet high up.