As a US citizen I have an SSN, file taxes every year on my foreign employment income, hold US-domiciled assets (stocks, IRA, 401k), and I have a ~15 year credit history with a decently high score.

Most (not all) HELOCs require that the property is titled to an individual (not LLC) and are full doc, meaning your employment history, income, DTI etc will be factors.

My son (age 24) however, is an independant contractor / self employed and is trying to buy a house. 

It's likely a scam anytime they sound like AI or the grammar is weird or they use the word "kindly", if they ask for you to respond to them off-platform, if they say their employer will pay for it, if they ask if the dates they've specified are still open (when they can clearly see they are open on your calendar), if they ask for a video of your property (they probably want to set up a duplicate listing with your photos and video and scam others out of money), or if they want a long term booking and they have just joined the platform and have no reviews (could have been evicted from somewhere else, and hope a newbie will let them in with no background check, they'll end up not paying or credit card is bad).  

Use umbrella insurance as an additional safeguard, especially if you opt not to form an LLC.Avoid S-Corp taxation for rentals, as rental income is passive and not subject to self-employment tax.

For all methods you would want to find a market that has strong employment rates, strong income rates for the market, and is a growing or an established market. 

Focus on neighborhoods near employers or schools, and explore slightly outside Austin for more affordable options. 

Also be aware of Unrelated Business Income Tax (or UBIT) on leveraged real estate inside of an IRA.Or better yet, since you are self-employed - go with a truly self-directed Solo 401k plan, which would be a much better option and will help you avoid the UBIT on this investment.

I’ve been researching options like offering fully furnished rentals for traveling professionals or executives, especially with major employers expanding their footprints in the region.

We will both keep our W2 employment for the near future just looking into getting an extra income or make our retirement easier.