The military is not a community worksite. They need to people prepared to do a job. Mentally and Physically. There can be no question. Gender or Transgender. If you can do it, then you can participate. But... there can be absolutely no question that you can do it.
Do your eyes work well? No?, then get out of this Pilot seat
Can this person lift 80lbs for 20 miles hikes a day? No?, then join the Peace Corp
Is this person mentally stabile without any prior history of depression? Welcome aboard.
Are they mentally stabile enough to follow chain of command under stressful conditions? No, then you cannot join the military.
I don't believe they need to reduce minimum requirements for diversity. That makes zero sense for war.
The big hurdle for Trans will be proving they meet the mental standards. The statistics are against them and the military knows it, and no one is doing studies to help them show otherwise.
I have major mixed feelings about females in the service.
One undeniable thing is the issue it causes with fraternizing, which is a massive issue in most branches (especially the Navy). I'm not discussing nonconsensual contact, either. People have a hard time understanding that the UCMJ is NOT civilian law and it really makes things difficult with sex is involved.
Personally, I have some issues with females being in certain roles, like you noted. If a job requires you to perform a certain way to do a task and they have to lower those standards for anyone, wrong answer. I have long disagreed that they have female vs male fitness requirements for fighting MOS soldiers, that's complete bullshit. Either a female can meet minimum male req...or she should not have a field/fighting role. End of story.
Lastly...and this is the most important, is the rights of everyone else.
Let's have a hard look at this trans issue. Males and females, at least in the Army, have shard facilities segregated by sex. It is grossly unfair, and honestly offensive that a biological male, with male genitals, will be force fed on females who group shower. I'd say the same about a F to M...but the reaction in something like a group shower would be vastly different.
Because I strongly doubt that they'd force-mix people like that, I'm guessing the trans-person would get special bathrooms, showers, etc and that completely breaks down the entire system.
This transcends "well, if they want to go serve the country, then let them.", once again people taking a horribly complex situation and making it sound easy, which it is not.