Hi, I’m a General Contractor, licensed Real Estate Agent, and land developer. Your ultimate end user in the USA is definitely a niche clientele. Having been approached by similar vendors in my home building business, I would certainly say a portfolio of designs would help set you apart from other designers and have buyers confident in your work. On top of creating recurring revenue you could “modify” the plans for clients for a fee.
In terms of growing your business I would recommend contacting contractors with a portfolio of your work as they are essential to fulfilling a buyer’s dream. I would create your ecosystem website then offer builders to be a partner on your site to build your plans. With that, you have your foot in the door and have opened your network. Then reach out to Realtors who have vacant land listings and offer to list a completed home with your builder partner and you have yourself a home run.
I wish you the best of luck with your business!