If military tenants are more your speed, you’ll want to ensure your properties are an easy commute to the military base. 

The few lenders I've reached out to in my area (local banks, credit unions, etc.) are only working with primary residence for Helocs or Cash Out refi.

I have had travel nurses, medical students, and military members stay for multiple months in some of my STRs. 

I have been working with a credit union to get a HELOC to allow me to tap my equity to buy additional real estate.

The other option I would consider is regional credit unions that hold onto mortgages/loans and don't sell them to the secondary market - they are "portfolio lenders" and have the ability to be more flexible with their terms.

Is there a bank or credit union that offer a loan for it?

Doesn’t fit your time frame but join the military.  

I’m active duty military and I’m about to enter my shore duty.

To focus on your area, research neighborhoods for trends, look for properties near military bases or colleges, and consider long-term rentals or short-term rentals in coastal areas and tourism hubs.

You could look into a cash-out refinance with a private lender or a local credit union.