The Nero show tests were done on a pan flat crit circuit, no significant hills. If you were riding with a lot of climbing, I expect the weight of a 40mm tyre would be very noticeable.

I love wider tires they're more comfortable and provides way more confidence cornering agressively. They usually feel slower but from my experience this mostly an imporession, the difference in speed is often negligable as long as you pick the correct tire pressure and you pick good tires. As an example, I recently switched my road wheelset (Continental GP5000 S TR 32mm) with my gravel wheelset (Schwalbe G-one RS pro 50mm) on my bike, while they felt way more sluggish when I'm looking at my time on section of flat smooth roads the difference is marginal. My gravel wheelset is slightly deeper 60mm vs 50mm so that might play a role, and the aerodynamic gains of the deeper wheelset might compensate for the slight rolling resistance penalty.

I don't think weight makes such a difference either. Between the 28mm and 40mm version of the Pirellu tire there is only 130g of differece, so with two tires it's around 260g, so around half a 500ml water bottle. You're own weight probably varies more than that during a long ride.

Yes the groupset on the bike is LTWOO GRT12, with 140mm rotors. The brakes are flatmount and they came with an adapter with one side that is 140mm and the other side that is 160mm if I remember correctly.

I originally bought the wrong T47 BB, it requires a T47 internal BB with shell width of 86.5mm. I bought this one: https://aliexpress.com/item/1005005997553550.html (model: B-T47-DUB(86.5MM)).

I just finished building mine !
I'm still waiting on building a new wheelset for this bike, so for now it's more setup for road rather than gravel but comes under 8kg.
I also haven't installed the integrated handlebar, I'm waiting on a bikefit before installing them.
Will send an update once I have both.