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.

Effectively offering more than 100% financing.  

Your referencing effective age vs chronological age. 

You can get the Bigger Pockets paid membership and use their Find a Market Tool to analyze.

Pull out a spreadsheet and go through the tools everyone mentions to run some comparables and take note of photos, amenities. 

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

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 have the tools and mindset that work with larger scale commercial so I would network until you meet someone who may let you run some of their deals with them and see what they look like in person.

I own a 30-unit community and found it much easier and more cost effective than the 1-4 family and flips that I was doing before that acquisition. 

@Monish Anand Bigger Pockets has an 'agent finder' tool.