Hi, dktravels85, I currently own the Sony a7rii and the canon 5d mark IV, i focus mainly on landscape and underwater photography. In my opinion, the best suggestion as to what lens to have for landscape is in my opinion, the Sony FE 16-35, despite its rather steep price tag, the focusing is extremely fast and for a F4 wide angle, the distortion is surprisingly small. However, as this is a ultra-wide, its usefulness will be very limited in other situations. Sigma's line of contemporary lenses give a excellent balance between lens quality and pricing, plus, with the mc-11 adapter, Sigma's canon mount lenses can be used with near 100% compatibility. I strongly suggest that you do get a mount converter, either the mc-11 or the metabones. Of course, as people before me have mentioned, the 50mm 1.8 is an excellent lens and will be a useful addition as well!
Good luck on your search!