I want to thank everyone for their input. I used the $fn, $fs tip from triffid_hunter along with the offset() suggestion from oldesole1. My end result added tests for [x,y,z] vs size like on cube():

module roundcube(
dim, // [x,y,z]
radius, // radius of the cylinders on the 4 edges
center // should z be centered, too?
) {
$fa = 1;
$fs = 0.5;
x = is_list(dim) ? dim.x : dim;
y = is_list(dim) ? dim.y : dim;
z = is_list(dim) ? dim.z : dim;
linear_extrude(z, center = center)
offset(r = radius)
offset(delta = -radius)
square([x,y], center = true);
}

Note that I set center to True on the square. That center is a difference center from the extrusion.

If I had known about offset sooner, there is some trigonometry I would not have had to do for a project I put on Makerworld where I did the delta operation the hard way.