If I understand your drawing, that is very similar to what we have. I think our peninsula might be 7 ft instead of 8. Is that the countertop doing the "waterfall" thing at the end of the peninsula? We actually have a cabinet panel supporting that end. There's no plywood or extra quartz under our overhang, which is 12" (and perfectly comfortable for us, contrary to the conventional wisdom.) We don't dance on the counter or anything, but I'm sure it's fine. Our contractor is very experienced and he hired the best countertop guys in the area (independently confirmed by our cabinet guy).

How thick is your counter? We have a similar overhang and were told we needed a support for the 2cm, but not the 3 cm (which is what we ultimately chose).

Yes, it will be "waterfall", and the edges will be mitered. 20mm thickness.

I read the ceasarstone installation guide and they recommend plywood support for 20mm countertop overhangs that are more than 8". And I don't wanna have plywood there I want the same quartz underneath.

On the other hand Viatera says "maximum unsupported overhang is 11" for 2cm material"

Silestone says 11" for 2cm as well. I wonder why Caesarstone is so paranoid?