I can't give you Army specific advice, but what I can give is general advice. Make sure you are in the best physical shape you can be before you go in. That way, training will be "easier" and you may have the ability to take on more things to make it more likely you get what you want. If you are in the top of the class, you have better opportunities than those at the bottom.

Most of your question would be better answered if you could find someone locally who has done the job (preferably recently) and have a face-to-face conversation. If you go airborne, you’ll probably end up at Bragg, but there is little call for airborne CI. The Army, like most jobs, you will get out of it what you put into it. If you make it a career, focus on your end goal, and work to meet that. It’s a moving target how to move up and excel, so there really isn’t a 1/10/100-steps to be good at Army, CI, or life-in-general. Find a mentor who is good at the job and can show you how to be good at the job, that’ll at least get you off on the right foot. Find your strengths, and rely on them, but always work on your weaknesses. This is a little general, I get it, but keep working hard. Don’t think you’re going to be James Bond, Jason Bourne, or any number of other movie or TV style agents, that’s just not how things work.

For anyone who may not know, 35L is now an entry level job. I know it wasn’t some years ago.