R6 with 300mm ef and 1.4x

I rented an R6 and took it to Hawaii and mostly shot it with the 24-70mm 2.8, 50 mm 1.2, and 85 mm 1.4 using the adapter.

I also do a lot of shooting with my 300 mm with 1.4x.

I’m wondering how feasible this is given how much smaller the mirrorless camera is?

I use an R6 with my 400/2.8 ad it works great. I try to not hold the whole rig by the body alone, especially at an angle, but I've always doe that, even with 1D bodies.