Thanks for all your advise and options but looks like i'll stick to carrying the 6d and 17-40. My bag when walking is about 15kg so not worth changing it if i'm only going to be saving a little weight.
You would be saving around 300grams to 500grams overall depending on the lens with an A7.
Even the best low light APS-C bodies like the D500 and 7D MK2 are actually heavier than what you got.
APS-C Mirrorless would save weight but again the better low light bodies are not very,very light either.
There are cameras like the XT2 which some full frame users have changed from,but again the issue is that they are comparable weight to the A7(!). This is why the first A7 had such an impact when launched - the vast majority of APS-C mirrorless cameras weigh about the same.
Also the basic issue,is with the smaller sensor cameras you are getting physically less light onto the sensor:
https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care
So a smaller sensored camera might actually not have as small lenses as you want in reality to equalise things(look at how massive the 18-35MM/F1.8 ART is for example).
Edit!!
This would be the smallest interchangeable lens camera with a decent sized sensor:
https://www.dpreview.com/products/panasonic/slrs/panasonic_dmcgf8
But whether it will meet your needs for good low light ability is another question as it has a m4/3 sensor.
With the collapsible 9-18MM(18-36MM equivalent) lens,it would come to around 400 grams. With the better 7-14MM(14-28MM) it would come to around 600 grams.
But again can't say how the image quality is when compared to a FF camera.
You would have to step down in image quality quite a bit in low light methinks,so there is only so much weight saving you can have sadly with a FF sensor and lenses.
Second Edit!!
Looking at one of the smallest dSLRs(mentioned earlier),the 100D weighs 407grams. With the very slow 10-18MM STM/F4.5-5.6 it comes to around 650 grams,and that will worse in all metrics than your current gear in low light(especially with the older generation sensor). With the larger aperture EF-S 10-22mm f/3.5-4.5 USM,the total weight is 800 grams.
The Eos M mirrorless cameras with the newer Canon APS-C sensors are over 400 grams too. With the EF-M 11-22mm f/4-5.6 IS STM again it comes to around 650 grams too.
But remember F4 on a FF camera is not the same as F4 on an APS-C camera either,so you will be giving up low light ability but the EF-M lens does have IS which helps.
Again I don't know how sharp these lenses are compared to what you have now though.