Trans people I know are very sure about themselves. More than some cis people in fact. I will let RAND Corporation rebut the rest of your speculation.[1]
The effect on force readiness is an important consideration in any military policy change. Many types of medical treatments can lead to lost labor and leave personnel temporarily unable to deploy. Given the small estimated number of transgender personnel and the even smaller number who would seek gender transition–related treatment in a given year, the study found that the readiness impact of transition-related treatment would lead to a loss of less than 0.0015 percent of total available labor-years in the active component. Even using the highest estimates, less than 0.1 percent of the force would seek gender transition–related treatment that would affect their ability to deploy. As a point of comparison, in the Army alone, approximately 50,000 active-component personnel were ineligible to deploy in 2015 for various legal, medical, or administrative reasons—a number amounting to around 14 percent of the active component.
A key concern in allowing transgender personnel to serve openly is how this may affect unit cohesion. The underlying assumption is that if service members discover that a member of their unit is transgender, this could inhibit bonding within the unit, which, in turn, would reduce operational readiness. Similar concerns were raised in debates over whether to allow gay and lesbian personnel to serve openly, as well as over whether to allow women to serve in ground combat positions. However, evidence from foreign militaries and the U.S. military has indicated no significant impact on unit cohesion or operational readiness as a result of allowing transgender and gay and lesbian personnel to serve openly or allowing women to serve in ground combat positions.
[1] https://www.rand.org/content/dam/rand/pubs/research_briefs/R...