ALL of my major problem tenants happened to have had a average (adults in the household) score below 625. 

Sub2 is a risky strategy for a long term buy and hold as the longer the deal structure stays in place, the higher the chance that issues will arise (like issues with the loan servicer, insurance, seller filing for bankruptcy, seller dying, seller realizing their DTI ratio and ability to buy their next property is negatively effected and becoming upset about it like in your case, due on sale clause being called, etc.).

You don’t need a judgement, but not sure how effective it would be:   https://www.rentrecoveryservic...

If you pay cash, you don't have a payment, but effectively you are giving up an income stream if the cash had been in a bond or ETF and earning you either interest or appreciation. 

I completely agree with @Ashish Acharya's points about the cost-effectiveness of a HELOC and the potential credit implications if it’s secured by your primary residence.

The tenant has not paid anything since April 2024, effectively staying in my property for free for four months.

I have checked with our lender and keeping this home's mortgage will not effect qualifying for our next house.4.

Point is the above comments there are many other more effective ways to meet the FINCEN stated objectives for tax evasion, money laundering and terrorism.   

In effect, you will end up paying tax on your refi cash later on, when you sell the property.

Currently you have a STR lease and can effectively move anywhere the job market takes you.