Hello,I am a Software Engineer and a Real Estate Investor working on a tech product for the real estate mortgage brokers.

I've come to realize paying down my mortgage is the best investment I could make.

The main reason I ask is because that is significantly below primary residence mortgage rates let alone any investment loans, if you can get rates like that I might need to make some changes on my end hahahTo answer your original question, assuming all else is equal long-term fixed rate debt is valuable. 

We just acquired an existing duplex in Fort Lauderdale and planning on tearing it down and build a new duplex instead.

Maybe you saw this from the Tico Times already, but it lays out mostly depressing details on getting a mortgage in Costa Rica: http://www.ticotimes.net/realestate/25-mortgage-ti...Please post back here if you find anything!

And your bank is not requiring any sort of paydown of the mortgage for a partial release. 

Sometimes, buying an existing property can be more cost-effective and less time-consuming and maybe require slightly less permits, etc. 

This allows you to access funds while keeping your existing mortgages intact.Second Mortgage: Explore lenders who offer second-position loans on investment properties, though rates will be higher.Cash-Out HELOC : While traditional banks often restrict HELOCs on non-owner-occupied properties, some portfolio or private lenders may offer HELOCs for investors.With $15-20K in liquid funds, look for deals where you can negotiate terms:Seller Financing: Negotiate lower down payments or interest-only periods.Subject-To Financing: Assume the seller’s existing mortgage while covering the down payment.Lease-to-Own: Lock in the purchase price while using rental income to build equity.

I have a 2.5% interest rate on the balance of the mortgage. 

Knowing your long-term plans can help you build a long-term financing strategy so that you are not just worried about purchase number 4, but also purchases 5 - 20+ (if you plan to continue to invest).I'm an investor and licensed mortgage broker.