Starting from scratch

The title pretty much says it all. For personal context – 19M. Ship date is Sept. 30th. Navy nuke. Sub vol. I’ve been with the same young lady (19F) for 3 years as of next month. Plans to get married once she completes this sophomore year of college. [(Before giving me the don’t marry young speech, just know that we have spent close to one year apart with minimal contact because she plays juco softball and I was working 2-3 jobs over 4 hours away. She helped me put myself through high school. (Same school & graduating class.) She showed unwavering loyalty even when I was homeless.)] I just got a $500 secured credit card with Navy Federal Credit Union. I’ll have 8-10K upon leaving for RTC. Going in as an E3. Receiving a 75K enlistment bonus. Broken down as follows; 25K after RTC, 16.5K & promotion to E4 upon completion of A school, 33.5K upon completion of prototype training. The plan is to STAR reenlist for the 100K+ bonus & promotion to E5 upon completing my first 2 years of service. Of course I have hopes of making STA-21 selection to earn both my degree and commission as an officer. However, I know selection is very competitive and I will focus on being the best sailor I can be. As far as my financial mindset goes, I live by 2 sayings. Manage the bottom line and let the rest fall into place. Take care of your pennies and your pounds will take care of themselves. Now for the questions specific to me & my situation.
National Guard vs Navy Reserves to cover gf’s jr/sr years then med school? If the education benefits are better with the NG, how hard is it to transfer states? She’ll have completed 2 years of college before she enlists.
BAH vs living on base? House hacking with a VA loan? Building a barndominium multi-family property with the bonus $ and hiring a property management company? Other investment ideas for the bonus $?
Cost efficient car recommendations? We were thinking Nissan Sentra/Altima, Hyundai Elantra, Honda Civic, or a Toyota Corolla/Camry.
Budgeting strategies popular among service members? Spreadsheet templates? Free financial services such as counseling/planning? How much of a difference does the tax advantage of deployments really make? States that fully exempt military income, would this include pension & disability pay? Couldn’t find a definitive answer. Is staying in a full 20+ worth it? Pursuing higher education while on shore duty vs pursuing higher education full time as a reservist? If not selected for STA-21, would it be worth it to switch to reserves and pursue higher education after the first 2 enlistments are up then commission? How could/do officer’s benefit when going from bachelors to masters or even a P.H.D? Sorry for the super long read. Thanks for any/all helpful responses in advance! God Bless!