I'm pathetic at looks and clothing. I have so few clothes, even I get embarrased about it at times! However, a string of pearls always looks good on any woman, and it can suggest formality (and / or age).
I do have pearls, they were a 21st birthday present, so I should wear them rather than have them sitting in a drawer. It's a bit Tory matron, though. no suitable emoticon!
I also wonder if you are thinking enough about where you are going to be when you dress. See, I would have thought that a minister who looks younger would be a good thing, good for encouraging young people into the church, if not the ministry even. But perhaps there are times when you could shift the emphasis in your clothing according to the majority audience for the day.
I usually wear a black or dark skirt or trousers and a black jumper or cardigan when I'm in church or visiting people. Pretty much standard "office wear", but maybe not as formal, now I think about it. A lot of people do comment on how young I look when I turn up at their door and tell them who I am.
I've started telling them how old I am, but I feel kind of childish doing that!
Personally I think it's nonsense. If you are good at what you do, that's all that should matter, and all this frivolous stuff should be consigned to the dustbin of history.
It is nonsense, but it's getting me down just now. 
On the other hand, and this will come as no surprise I am sure

, I would opt for the more confrontational approach. Find a way to challenge their pre-conceptions and get them to acknowledge it. I think you are much more ambassadorial than me


so I am sure you can find a way to do this in a non-challenging manner.
I don't know how!
Now to the bit I hate! Maggie Thatcher did a lot of personal work to improve her likeability (didn't work for us up here, I know I know!!) But one of the things she did was try to lower the pitch of her voice somewhat to impart gravity. It is not uncommon, apparently, for aspies to have a more highly pitched voice. And being a woman, you also start from a higher pitched base. Bringing the treble down a bit and increasing the bass might help a bitty?
Aaaaaaarrrrggghhhh!!!! Not Maggie!!!! Back to the Tory matron!
If I do the deep voice bit I'd sound like Kirsty Young and all those other Scottish female presenters. They do all have a certain sound which irritates me somewhat. I don't think it is my voice though, as sometimes I'll have spoken to someone on the phone a few times, and they're still surprised by my youthful appearance. 
Good luck with it though.
Ta!