I have the SmallRig L-bracket on my M6 mkII, and I tried just for the fun my EF-S 15-85mm on it, and I found it SO front heavy, I felt not comfortable holding only the grip of the camera with so much weight on the lens mount. So I understand what you mean, and the 17-55 f2.8 is heavier than the 15-85.
I think that Canon EOS M camera are small plastic camera body that have been designed for small and light lenses, and I think that Canon dslr Rebel are more sturdier to withstand heavier lenses. So when you shout hand held, as long as your left hand hold the lens and the right hand hold the camera, there's no problem, but I'd not put this camera on tripod with the 17-55. If you do it, I'm sure it not gonna break tomorrow, or next month, but you push the limit. All that stress on the plastic structure of this camera, to me it's like you buy a pick-up truck that can tow 10000Lbs, and you constantly tow 14000Lbs. It'll last for a while, but everything are going to wear up very quickly.
So my solutions for you are =
Use the EF-M 15-45 on your M6, and wait for the EOS R7 to use your 17-55. I expect that R7 will come in a body like R5, so it gonna be able to withstand heavier lenses than M camera.
But if you really want to use your 17-55 on your M6, and put these on tripod, here's what i'd do. A little bit of DIY. Take a Dremel rotary tool with a grinding bit, and grind a little bit of material on the tripod's foot of the EF-EFM adapter, and grind a little bit of material on the ARCA swiss plate of your SmallRig L-bracket. You'll not need to remove a lot of material, to be able to correctly install the adapter on the M6. With that little modification, you'll have a very sturdy and reliable set-up, which will last a very long time.
But that's just me... You do what's you want...