I don't see the issue here?
We planned ours in 15 months from engagement to wedding day
Church was available on our chosen date, as were the venues we wanted. And yes it was a summer saturday and a full scale trad wedding.
18 months should be absolutely plenty.
IME there is a bit of a rush at the beginning of the planning - when you work out number of guests and your preferred date, book the venue(s), caterer, and maybe the photographer or a band if it's really important to you. Then everything else can wait a while. Then there's another busy period about 6-8 months before the event which is when you need to sort save the dates, dress, bridesmaids, invitations, florist, hair etc. Then another quiet bit. Then another busy bit in the run up to the wedding when you send invites, sort table plan, choose menu, and confirm and pay everybody...
Of course many of these things can be ditched to simplify life! But what I'm saying is 18 months is plenty even if you want "all the trimmings".
I think that when people take longer than 18 months it's usually for financial reasons or weight reasons ie they need longer to save up or lose weight? (not that anyone needs to lose weight to get married but some want to).