I strongly advise you to get your primary residence taken care of FIRST.

Since there is a co-owner, you would have to purchase the property and it is really hard (if not impossible) to pull a HELOC as the basis for the purchase (since it is not your primary residence). 

In some cases, after running the numbers, I’ve even faced potential negative returns, causing me to miss out on promising deals here in South Florida.That brings me to my question: I currently own two homes—one is my primary residence, and the other is a rental property.

If you can get a better primary residential HELOC rate than you can a hard money bridge loan I would say yes.

My primary strategy is Fix and Flip, and I bring a DIY attitude to every project.

Which may also be a reason to hold on to your current place (appreciation).Second, have you been in your primary at least two years.

Currently, I own my primary home and a rental in San Antonio, TX. 

Will you be keeping or selling your current primary?

However, consider STR seasonality, local regulations, and management demands, and ensure your equity loan and new primary residence costs fit comfortably within your budget.

While both involve finding and properly securing a site and bringing it to full entitlement, the ground-up process includes the land piece but gets way more involved and complex to execute.The full "Monty" for ground-up development needs to include financial literacy, an explanation of metrics, and how they work.