I wouldn’t recommend a phantom center for reasons stated above and also because it sounds like the L/R speakers will be spaced pretty far apart and you’ve got people sitting off axis — all of which cries out for a center speaker. SVS offers a high-performance HT speaker package, and even adding a pair of their SB3000 subs (and you really should get a pair) they’d still only be around $6500 (+ tax), and they offer a 45-day, risk-free in home trial including shipping both ways.
https://www.svsound.com/products/ultra-tower-surround-package
https://www.svsound.com/products/sb-3000
If they prefer a wood veneer a similar system could be assembled with Aperion Audio speakers (but I’d still go with the SVS subs)…
https://www.aperionaudio.com/collections/verus-speakers
They’d likely be thrilled with either, and with the $$$ they’d save they could take a really nice vacation. Hope this helps, and best of luck.