I don't think you understand why it's a marksman in the duo lane.
It's not a "the team needs AD damage, may as well be bot", it's a "Marksmen, unlike mages, scale primarily off of gold and not levels, and so the shared xp aspect of laning with a support is not a problem. (Similarly, conventional supports are so chosen because they are inherently useful even without gold/levels.)".
The vast majority if not all ap champs would end up outscaled by their solo lane counterparts due to being a level or two down, and so be relatively unimpactful. There are lots of good initiation options for the support role, and if you have 4 other members that are poor initiators already, maybe your comp is better off focusing on splitpushing and/or getting picks rather than trying to initiate in the first place.