20mm for Canon FF. Voigtlander pancake?

[Ammo 7]

I'm traveling around for a month with my 6D. I have a 40mm f2.8 almost permanently attached. At around 900 gr. with lens, carrying it around my neck all day is not a problem. My 5D2 with a 50mm f1.4 was a bit of a lump. The only time the 40mm comes off is to fit the 17-40. However the image quality of the 17-40 is pretty mediocre in comparison the to 40mm prime. The 40mm STM pancake lens, although cheap, is pretty amazing. It is sharp, has low distortion and CA and focuses fast enough for me. Better than both my 17-40 and my 24-105 at 40mm.

As I am using the 17-40 at around 17mm most of the time (where it is at its worse in my opinion) I was wondering if I could get away with just a 20mm lens. Anyone got a Voiglander 20mm f3.5? I understand that you have to manually focus. Not a problem. Does the diaphragm automatically stop down on a Canon or is that manual too?

As I will be traveling more I want something light to carry around as the 20mm will be probably be for just occasional use. The alternative is to perhaps get an old Olympus OM 21mm with an adapter. Lightness, IQ and small size is important. As I will be traveling with just carry on baggage I was thinking 6D, 20mm, 40mm and 85mm and Kenko 1.4x for a bit extra reach for the 85mm should I need it.

Any suggestions or experience of the 20mm Voiglander gratefully accepted.