Thanks for you answer. This is not an extreme cross-chaining as the position on the sprockets is the 3rd one, out of 9.
The convenience for me is that I can start from on a complete stop on the 3rd position and then shift to higher speeds easily, from the 3rd to 4th, 5th, etc. It would be a little annoying to have to change the front gear and also the rear gear to achieve this.
What do you seasoned cyclists do?
Maybe it is just a simple adjustment, because the rubbing is not constant, but only when I put lots of pressure on left crank, like when climbing, which makes me think that the BB set is not stiff enough.