I wouldn’t add lactose. Your mash profile could be your answer, I would scrap the first two steps and make the third step (now your first) 65C for 40 minutes followed by 70C for another 40 minutes, finishing with the mash-out at 77C for 20 minutes. Try that in your brewing software and adjust the temp or time of the first two steps to get what you wantYou mean I shouldn't believe what I read on the internet!? Probably just as well I didn't set out to make my own candi then.
I did have a little read of Palmer and now concerned at his comment that golden syrup can taste quite salty, so 1.2kg in the beer might be too much. I had a tinker and thought about reducing the molasses and using 500g demerara sugar but then noticed my FG was predicted to be 1.005 max which is probably a bit thin for what I'm aiming for.
Any thoughts on what I could do, perhaps a small amount of oat or lactose to get up to around 1.010? I'm thinking oats as I'm asking a lot of the yeast, so if it doesn't fully attenuate and I've added lactose, I've got a sweet beer at the end.