Thanks for the image. Nice looking boat.
My suggestion would be to flow the existing upper rub rail area onto the new hard top. That is having the same small fiberglass height above and slightly larger below that rub rail.
Where the new hard top meets the existing structure, make the new hard top edge a horizontal "U" shape, w/ the lower part longer to follow the existing lines.
For appearance and functionality the new hard top should stop inches short of the transom, ending following the existing transom line, or slightly longer for directing rain overboard.
For appearance I would have aft side support panels, w/ the mechanical strength buried inside. The "easy" approach would be to have ~one foot wide vertical panels. It may instead look better to have those support panels angled forward, matching the aft window panel angle. If the new side supports are angled, then the new hard top must be strong enough to support that aft cantilever hard top area.
Don't forget to include large diameter buried wire passage conduits for future updates.
The new hard top needs to have a solid top surface. The appearance would be best with a solid bottom surface, instead of being able to see the mechanical structure.
My assumption is that the new hard top is not for walking upon. If walking is required, then strength for that load must be considered, plus side railing to retain passengers, plus a non slip surface. It is possible that passenger mass at that location would create a very adverse handling effect.
Noticed the boat doesn't have a radar, probably for fixed bridge clearance. This new hard top could be designed to have a handy manually raised / lowered post w/ a radar on top, placing the radar above the heads of people standing up top.
The new hard top could benefit from one large or two smaller (side by side) opening sunroof panels w/ sun shades.