Why not a 15mm F4
(to reduce weight and price) non-L (to reduce price) prime rectilinear lens
that can take filters
? IQ should at least be better than the current 20mm F2.8 (close to 17-40 IQ stopped down to F8). If they price it just a little bit more expensive than the samyang 14mm, I'd take it in an instant. It doesn't matter if it's MF or AF.
The brand doesn't matter as long as those other important features are available.