to get a round object you need atleast a hexagon like profile, of course the more verts the more round it will be( to view this effect create a mesh circle and in the creation options for that adjust the number of verts in the circle you’ll see 3 is a triangle 4 square and so on through the various basic geometries, 16 starts to look like a circle and its defult is 32. also turning on smooth shading in object mode would help it look round, to slect an entire line of verts edges or faces is alt click ctrl R to loop cut and slide. using these methods would help you to make it more rounded
Select the long edge, and ctrl B to bevel. See the wiki for details!
If triangle limit is 350 you are already well past that after you apply the mirror modifier…
To keep the poly count low, you basically need to pull the central geometry out to give thickness and leave outer bow contour as single shape. You need to reduce segments as well. Here is one I made with 316 poly count.