@KaiWei Tang
I applaud you for taking some important steps toward purchasing your first property. It sounds like you are working on educating yourself and living within your means.
I'll echo what several others have said...if you have the ability to qualify for a conventional mortgage to purchase an investment property, I'd recommend making that your first option. A conventional mortgage will have a more advantageous interest rate than a DSCR loan and offers some other benefits.
You mentioned an FHA loan, but that mortgage type is intended for a primary residence. Since you won't be moving to San Antonio for 3-4 years from now, purchasing a home here in San Antonio with an FHA, VA, or USDA loan won't be an option.
If you'd like to take some time to review your specific situation and run some financing numbers/scenarios, let me know. And I'd definitely recommend connecting with @Hamp Lee III...he's an awesome Realtor here in San Antonio, is active here on BP, and is also a real estate investor himself.