I think most people would agree that solving this for fairness to the benefit of the majority (i.e. the cis women athletes) would be really easy. It's the addition of the goal of inclusivity of a slim minority that complicates it. So it's really how you balance those two goals.
Those suggesting the open category is an easy fix places almost all of the importance on the fairness part, and little to none on the inclusivity part. This is fine as long as you are honest about it. Saying that "Well, cis women can choose the open category as well" is just disingenuous.
If you look at sport broadly, I would say there are three parameters that should guide how those two goals are balanced.
1) The level of the competition (i.e. elite vs. amateur). The more you approach the elite level, the more you have to target fairness.
2) The age of the competitors. At lower ages (i.e. pre-pubescent) the physical advantage of being born male are more limited.
3) The nature of the sport. The higher the degree of physical traits required to succeed, the more you have exclude - especially when physical contact is also involved.
The problem is that one of the goal shouldn't be one unless you just get rid of all gender/sex divisions.