I’m looking for lighter gear and recently switched to the M5 and 18-150 as my landscape and walkaround set-up. I also shoot wildlife and birds with my 80d and 100-400 ii, but that combo is pretty heavy to handhold. I tried the 100-400 on the M5, but it was ridiculously out of balance. Thinking of switching the body to Sl2 or t7i, both of which have a bit more meat. What input do you have? Do you forsee any problems with either of these bodies?
If you haven't tried one, I would recommend looking into using a monopod. The SL2 doesn't have an AF system for shooting moving targets with telephoto lenses so the only viable option is the T7i between your two suggestions. The T7i saves you 7 ounces over the 80D which isn't much considering your current rig using the 80D with the 100-400mm is 82 ounces.
A monopod will take bulk of the weight when you are shooting and with a quick connect plate you can mount and dismount the monopod easily if you need more freedom of movement. The 800 lb. gorilla in your combination is the lens and a change in camera bodies isn't going to do much to reduce weight to any degree. Also, an added benefit of a monopod is it will help a good deal in steadying the camera and will probably give you sharper photos in many situations. It would also increase your ability to take sharp photos in less the ideal light.