The BT whole Wi-FI should handle M500 easily.
I have M200 and it shoots to 220mbs very fast. But you can't split the band on this mesh so the system choses the strongest band.
I chose 2 pack over the 3 to minimise backhaul traffic. The 3rd disc may need to hop on the 2nd disc for backhaul. Maybe best to connect 1 disc and test the speed before adding the 2nd and 3rd.
Forgot to ask if you getting 500mbs wired direct to hub3.
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M200 Oomph - 5GB SIM + 3 extra SIM family plan - BT whole Wi-Fi - Sky Q
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Don't cry over Virgin price increase instead speak to retentions team and negotiate a better deal.