I'm on the road without a lot of time to respond right now, but I'm happy to answer any specific questions. However, I'll say off the top that I know exactly zero about TSP or any other federal programs. But I will say kudos to you JK for taking things into your own hands. Investing really isn't that hard, it just takes some time to build your confidence and do some research. But as engineers, it's nothing we can't handle. Doesn't make much sense to me to pay someone for a managed account when you can easily replicate it on your own, for a couple hours a month of your time, tops.
Pick some low fee funds or ETFs (stay away from mutual funds with high expense ratios because they eat into your returns) and you'll be fine. The main thing is you're starting early and have the magic of compounding interest on your side. Even if you make a few mistakes at first, they'll be more than made up for by the compounding over time.