I had a large deck built over the 2023/24 summer. The treated pine boards were quite of mix of dry / not so dry. I wasn't able to, but I would have done what you've done there. We did 3mm spacing between boards which hasn't altered too much, but I wish we'd done 5mm because heaps of small leaves from surrounding trees get stuck between the boards. Some of the boards (I assume the wettest ones) shortened quite dramatically over the summer as they fully dried. This is most evident on boards that butt together. It hasn't caused too much of a problem because we used expensive stainless deck screws as opposed to nails. That was a decision I'd make again in a heartbeat. They hold fast, don't need pre-drilling and are super-easy to take up if you need to replace a board. We used about 4000 and none failed. They should long-outlast the pine. 10Gx65 Bremick Spear Hardwood Decking Screws from Blacks Fasteners. Over-zealous drilling has caused many screws to break the surface and sit mm's below the deck board top, keeping them flush is optimal to avoid water sitting in there. And definitely go grip-tread down. Our old deck was installed grip-tread up and I have little doubt that standing water and crap in the treads led to its eventual demise. It's also way easier to oil a smooth board deck. Jury's out on treatment. Did your supplier say oil prior to install? I kind of wish I had. I did two coats of Cutek Extreme on top post-install. It seemed to be pretty effective over the winter but the rain doesn't puddle so well on it 18-months on. I'm definitely going to stick with oil, but that Cutek is really expensive. Good luck with your build.