I want to add additional career branches to the existing athletic career.

I find if do the following example1.png I can successfully make the Investor career branch an option for the athletic career. I then create a copy of the Investor career track and change the id as per examples2.png and example3.png. The intention is to use the investor branch as a basis for the athletic branch that I want to create. However, when I load up the game I find it is although the athletic career no longer has any branches. Can anyone point me in the right direction of what I am doing wrong here? Is there an additional step i need to take because I am trying to add a new XML file? I'm creating the package file from the XML using S4PE.

I don't know how to build a career this way, but from what I can see, you might need to change the Business_Track3_Investor to Athletic_Track3 at the top in the 3 png. Also, did you adjust any other tuning files, such as the corresponding Sim Data XML file? It doesn't show if you've created the tuning for career levels, either. I think without seeing more of what you've done, it might be difficult to help.
I find using Sims 4 Studio helpful because you can see all the required files once you Extract Tuning. You can look at them in S4S, and make sure you have them in your s4pe setup.
Careers are very picky and it's best to have it completely created before testing, if you haven't done so.
Personally, I would use a different program to create the career from scratch. What you're doing might override the current athletic career, unless that is your goal. Zerbu's Mod Constructor might be a better fit. Obviously it's up to you what programs you use, I'm not saying anything about that, just a little advice.