The primary thing you're going to run into is whether or not a lender, at purchase of the property, will allow there to be a secondary lien on the property.

Im basically looking for advice on how i can save as much on taxes as possible.Part of the property is your primary residence, which does not qualify for 1031 Exchange treatment.

I own my current primary residence & have about 140K in equity.

I have 880 credit score, decent savings, and no other debt except for 300K mortgage on my primary residence (worth $1.5 million).

There are not traditional DSCR lenders that would do that.

I would look at the DSCR loan as the first option.By the way, I suspect you are going to be disappointed by any refi appraisals   ADUs typically appraise poorly   JADUs can lower the property value Vs having the space as part of the primary unit.  

My primary residence is an apartment I rent in Brooklyn NY.

For the son to qualify for the $250k exclusion, I believe he would most likely need to meet the ownership and use test, meaning he needs to own and live in the home as his primary residence for at least two of the last five years before the sale.

In this case my plan is to use a HELOC on my primary residence to purchase the property in Puerto Rico.

We have our primary In a trust but thinking if it's a good idea to buy the rental in a trust as well.