Do you need the roasted barley?
Or rather, should you be trying to make a Tim tam beer - where you focus everything on getting as close as possible to Tim tams - or a Tim tam stout - where you're splitting your focus between a stout and the Tim tam thing?
The choc malt will make it dark anyway.
Maybe check whether the fat from any chocolate addition will have a negative impact on the beer, especially with head retention. So maybe stick with the nibs, if you're going down this path.