I would try to learn to meter the light, so you fall into the habit of subconsciously adjusting your M6 as the light changes. That way, you are not taking a fresh meter reading when you lift it to your eye. You can meter, as you compose, but it is slower and it really necessitates learning to be comfortable adjusting the shutter speed when the camera is to your eye, likewise the aperture ring. This should help speed up your usage of your M6 when you have a 50mm mounted. Likewise, with a 35mm it should be quite nippy, as you are already quick to focus a 35, and once you can set the exposure the same way, shooting should be fast.
If even with the above, you find manual is still too slow, and you need aperture priority, then I would consider an aperture priority body like the bessa. If going down that route, consider the R3, which with its higher magnification finder may be a good compliment to your M6, especially for mounting a 50mm on.