You’re able to learn the basics of a real estate investment with lower risk and build equity at the same time.Because you're in the military, you'll be able to enter a property with 0% down which means you can leverage your money way more effectively. 

Local banks typically offer lower interest rates but might require a solid track record and detailed financials.

I would just take into account that a lower down payment will mean a higher monthly payment so be sure to run the numbers conservatively in order to ensure your STR revenue is able to carry your monthly payment with this higher mortgage cost. 

I would like to have lower monthly payments should I make a big lump sum payment towards my principals and ask my mortgage company to adjust it or refinance it or which one is the best ? 

This implies residential units can be built at lower costs and provide better return. 11) adding an ADU to SFH can make the SFH fall under rent control.

BoA is fine for this sort of setup.. but consider a local credit union in San Diego for lower fees and simpler sub-accounts from what I’ve heard.

Seems like it's in a lower end area so if it is filled you will have tenant issues and turnover cost.

If I have lump-sum free cash, should I just apply it to my mortgage to recast so that my monthly goes lower?

Some of the costs quoted shock even seasoned flippers as they are lower than even material are in many markets. 

When you researched markets like Austin las Vegas Miami, those markets have higher land cost and are more cyclical in nature tend to have larger price swings compared to regions with lower land costs.