Changing level of difficulty of assignments for a course based on average class performance
I would suggest that you step back and ask yourself why you are giving these assignments. Some people give out assignments purely so that they can grade students and then they think about fairness of grading, etc. 

But, if you consider that it is through reinforcement and feedback that students actually learn then the solution becomes clearer. If you consider that the human brain works by actual physical rewiring - connecting synapses - then you see that students need these exercises to make the material clearer in their minds and so that they can move the ideas from short to long term memory. This is discussed in the book The Art of Changing the Brain by James E Zull. It is pretty fundamental to learning theory in general.

Thus, the exercises should be there to enhance learning, primarily. 

So ask yourself whether the exercises you use are enhancing learning or not. Or are they just a stumbling block that some students can't get over? If it is the latter, then you should use different exercises that better enable learning. 

My goal was always to teach every student. If I had a reasonable number in a class and if they were willing to do what it takes to learn, then I could be successful. But for many students it takes a lot of practice to come up to speed, as well as a lot of feedback. 

But my goal was never to just deliver information and use what you want of it. Wikipedia does that job pretty well. 



However, for your weaker students - those who are willing to do the work, anyway, there are other options. If your current exercises are good in some ways and you don't want to drop them, you can provide additional exercises for those who want to try them. These might be chosen to better enable good performance on the current set. But you should also provide some mechanism for the students to get some feedback on these new exercises. From you is probably best, but sometimes peer-review might be sufficient. Or even set up study groups among the strugglers.