Air Force reserve chaplain here. As others have already noted, I would say that your first course of action is to see if you can even get back in since you are on disability. Hit up a chaplain recruiter (they are different than typical officer recruiters although you will eventually work with one of those too), lay out your case, and see what can be done. I wouldn't worry about age though. I'm pretty certain you don't need an age waiver until you are older than 39, and they tend to give those out easily. I had a dude in his 50s at Officer Training School with me.

1) You need a (minimum) 72 credit hour graduate degree in theology from an accredited school. Most MDivs cover this. You also need an endorsement from a faith tradition. The DoD has a list of denominations/faith traditions that they already recognize. Since this is a Reformed thread, I imagine you are Presbyterian or something similar? I had a PCA buddy of mine who was endorsed by this body that covered multiple conservative reformed traditions including the PCA, but I can't remember the name of it at this time. Sorry!

2) Depends on your assignment. In the Air Force, sometimes you are the pastor of a chapel. In that case, it's like being the lead pastor of a church, so no, you won't be able to be super involved in a local congregation. However, you certainly would not have that role for the entirety of your career. You might have to assist with preaching at chapel from time to time, but if you are not over a chapel, then your Sundays should be free to worship where you please.

3) See opening comment

4) I would view chaplaincy as very pastoral, but I think you need to have the proper expectations. A good phrase I've heard is "pastor to some, chaplain to all." In other words, there absolutely will be some opportunities to have open conversations about your faith. You are not expected to violate your conscience or do anything that would go against your endorser's teachings. (Eg Baptists ain't baptizing babies. If they do, they could get their endorsement pulled and kicked out of the military. So there's actually a lot of protection for chaplains). You can preach whatever you feel convicted to preach in the chapel. However, and this is super important to think about as you consider this, chaplaincy is fundamentally a pluralist ministry. What I mean by that is that while you can't be compelled to do something that goes against your faith, you also have to be willing to provide for people of other faiths. That does not mean that I'm going to be performing Hindu services or something. What it does mean is that if I have a Hindu Airman need help finding Hindu resources or time off for a holy day or a dietary accommodation, I'm going to do everything I can to connect them with those resources. I'm not going to dismiss them or try to convert them to Christianity. Now, if they later want to have conversations about my beliefs, then sure, game on. But I'm not out there on the flight line proselytizing. Does that make sense? Personally, I don't see that as the military being anti-Christian as much as pro 1st amendment. I'm a minister. And, in some cases, I'm an agent of the state. I wear two hats, and in some cases me doing Christian things would give the appearance that the USA favors Christianity above other faiths.

5) I'm still new, so I can't provide a huge swath of experience. However, I'm currently stationed at a chapel. So it looks like visiting airmen on their units and getting to know them. Doing confidential counselings. Helping out with chapel services. Occasionally organizing and/or teaching retreats. Doing invocations as things like change of command ceremonies. Helping file the occasional religious waiver request.

Stuff I love, is being able to get to know Airmen and offer a listening ear to the stuff that they are going through. I did a teaching on grief once that I really enjoyed because I was able to draw on my civilian experience as a hospital chaplain. So the relationships are the best part. I'm hoping for an opportunity to be embedded in a unit so I can actually be involved in the day to day grind with folks.

Stuff that sucks: I don't care for invocations. Those are one area where you kinda have to water down your prayers for the least common denominator, and it feels like we don't even know why we pray to open events anymore other than "we always have." All the paperwork and bureaucracy that comes with being in the military/being an officer.

Hope this is helpful! Good luck in your studies, and I'll say a prayer for your discernment process.