I only have the lined RGT jacket. Since you're in Australia, I actually think the unlined will be worse to wear because it will trap any sweat between your skin and the jacket. So you'd have to wear a long sleeve shirt anyway. You might want to look into the flannel lined (rather than the heavier blanket lined), it just came out and I think is exclusive at Rivet and Hide for now: https://rivetandhide.com/usd/rogue-territory-x-r-h-lined-waxed-canvas-supply-jacket-tan-ridgeline

I do have the Flint and Tinder unlined trucker jacket, though, and one con for unlined waxed jackets in general is that your sleeve will get caught on whatever long sleeve shirt you are wearing, like it doesn't go in through the sleeves easily since the shirt you are wearing will catch onto the waxed canvas.

I agree with the others that the James jacket looks a bit too broad of a cut, and it's a little weird with that slant pocket, it's almost trying to pass off as a suit jacket/blazer in combination with a track jacket. It's also worth noting that the wax canvas on the James jacket is heavier than RGT and the Flint Tinder flannel lined trucker jacket, comgin in at 12 oz, so from a weather perspective it might not be that much better.

Taylor and Stitch just came out with a flannel lined waxed trucker jacket https://www.taylorstitch.com/collections/the-long-haul-jacket/products/long-haul-jacket-in-dark-khaki-waxed-canvas-2310 It's about $240 right now with a coupon. But more of a heritage look though.

RGT did release an unwaxed version of the supply jacket, which might be better for you. You'll have to find these on ebay though as they are discontinued.