Drill bit and screw for counterbore with plug

I need to attach seat slats on a white oak garden bench. I have a question about screw selection and countersink technique.

I was intending to use square-drive stainless screws with a solid shank and course threads, and a slightly larger pilot hole on the slat boards than the rails in order to accommodate the solid shank of the screw.

Most of the articles I saw that discuss wood plugs talked about using flat-head screws with a v-shaped countersink that matches the angle of the head.

One article, though, said to use a brad point drill bit to create a flat bottomed hole drilled to the right depth. Beyond that, it did not discuss the pilot hole or the shape of the screw head, but it would seem like a pan head or pocket screw would be better for a flat bottomed hole.

Any suggestions? My main issue is that I want to avoid tearout on the surface of the slats.

One other question is whether there is any reason not to put a small amount of epoxy in the pilot holes in the rails before inserting the screws. The bench is going to be subject to rain, etc. I have seen that technique used for installing hardware on boats to keep moisture out.

Use a Fuller bit for creating the hole, countersink and counterbore. They can also supply the plug cutter to precisely match the counterbore. I have been using their products for decades and can attest that they are without equal.
http://wlfuller.com/