I've built an 818 coupe and solved the A-pillar problems, but I also don't see exactly what you are trying to do. Can you post a pic from a little further away?
Also, Mechie3 posted a comprehensive thread recently that shows many techniques for molding and finishing fiberglass that was very handy for me when I fixed the A-pillar gaps.

https://thefactoryfiveforum.com/show...-and-Materials