We are doing a new kitchen with antique white shaker cupboards & drawers and I'm thinking a painted, pressed metal splashback behind a free standing 90cm heritage style electric cooker and the subway tiles elsewhere. We are doing an oiled natural shaped huon pine slab for the main benchtop.

I'd pick the cupboard white first. Pick what you love. Then get the wall colour tinted to match the cupboards. Nothing worse than mismatched whites. Either match them or go a contrast on the wall.

I just googled & found this site, which quotes cost of R2.5 75mm. there are other suppliers as well. What was quoted to you for cost of roof installation, and was it itemised so you can see exactly what thickness the foil-backed insulation builder will use? I'm guessing the thinner (lower R factor). So, the new quote should reflect a credit for the cost of previous insulation, since it won't be used, then cost of thicker insulation added. Does that make sense? If you are unsure, best to speak with the builder. https://www.pricewiseinsulation.com.au/product/heavy-duty-fletcher-permastop/

What is the length of your kitchen island bench? I would suggest it depends on the shape of the pendant lights, but usually they are installed 900-1000mm apart. So in general terms, 2500 bench have two, 3500 bench have three pendants. It seems like you have enough colour going on already. Have you thought about brushed chrome. This would then match your rangehood, fridge and other appliances.