The biggest lessons I've learned about child development is that it is a process over which we have very little control.
I absolutely understand why it is appealing to believe you can keep the child at grade level. I also appreciate the value in trying to "go wide and not deep." I understand why parents are wary of anything that seems to take away from having a full and long childhood. I understand these sentiments because I voiced them all regularly when we decided to start homeschooling our five year old kindergartner who is now a teenager.
Turns out that this process didn't evolve according to my agenda. The pace of development was startling. Even with very loose unstructured academics with very little parental direction, academics FLEW faster than I ever could have imagined (even faster than we were advised they would). This happened even with going very wide with diverse and time consuming interests like music. Kids are all different of course, but our efforts at trying stuff like using grade level materials like Story of the World resulted only in frustration for all involved.
For me homeschooling is 100% about having a happy child. For my gifted kid being able to have challenge and let his brain go at the pace it wants to go is at the very core of who he is and it is fundamental to happiness. Further, looking back I now see he was wise to allow himself to spend some time working at the outer edge of his ability because in that process he learned essential lessons about handling frustration and perfectionism which is a positive thing to have happen before adulthood.
So, my two cents, sure try to go wide, try to encourage a variety of interests, don't push or fuss about asynchronous development because it is normal. But, keep a close eye on happiness and allow for the possibility that the pace at which the child is happiest is not the one you would choose.
