I am concerned that this would be abused.
Somebody who is going through a transition may not be physically capable, and may incur huge medical expenses. It seems to me that enlisting in the military would be a good way to have Uncle Sam pick up all of these expenses.
If the requirement is that they be able to serve openly and express their gender identity, it would be hard to deny them these accommodations. Unfortunately, this comes at the expense of military necessity.
The military is not an entitlement program. They have a mission, and mission should come first.
Unless you can change that one pesky chromosome, it’s impossible to truly transition.