Atlantis Halo or ECS Evo

Go with what's local. If you're in Sydney go 11'2" ECS Cruiser. If you're in QLD or northern NSW go 11' Halo.

For flatwater you want long and narrow with not too much rocker - so a lot of people myself included find the longboard shapes 11' and above track well in a straight line without too much swapping sides and decent glide. My friend, a longboarder with similar height & weight to you uses a Naish Nalu 11'4" x 30" x 180l for flatwater but he finds it still a bit tippy in the surf so 11'2" x 32" x 200l is going to be more forgiving and fun in small surf. The Naish budget board for that purpose would be the 11'2" x 32" x 220l Quest.

I learnt to surf on my friends 11'4" Nalu and now surf a shorter Evoke badly, but the 11'4" Nalu and other longboard shapes are more fun for beginners in the surf and also work better on flatwater (just don't put in the side fins). I would only go 10'6" if the balance tipped to the surf side - and even then you'll have more fun at the start on the longer higher volume board.

Edit: ECS is local to me so I've seen a few of their boards up close and they look great. Since I was in Northern NSW I got an Evoke on sale, but shape aside I prefer the matt ECS to the glossy Halo.