advice on off brand wide angle alternative to canon 17-40 wth canon 6D

Try the Samyang 14mm f2.8. Even if you keep the 17-40L, 17 mm still isn't superwide. So a prime gives you more sharpness and in this case a lot more on the wide side I still have to try it but I read lot's of great reviews about it and it is next on my list.