The model you should go with will ultimately depend on what the group is optimizing for (cashflow, appreciation, tax optimizations, diversification, and maybe even a little bit of social value doing something fun, potentially lucrative, and unique w friends).  

I would stop short of saying I found the inflection point of optimality and being pragmatic. 

Six figure income is still impressive for any residential SFR..optimally rates improve and so will rates of return with a refinance.

Focus on optimizing the current fourplexes (improving rents, reducing expenses).

@Nick Am Setting up a management S-Corp (LLC-B) to manage properties owned by your LLC (LLC-A) can provide tax benefits like retirement contributions, payroll tax optimization, and deductible expenses (e.g., home office, health insurance).

I am also looking for tax optimization as the business will require cash reserves and is a high risk business from a liability perspective.

Before committing to paying off the property, analyze whether there are ways to optimize revenue, such as better marketing, adjusting pricing, or upgrading the property to attract higher-end guests.Return on Capital: If you pay off the mortgage, the “return” is essentially the 7% interest you’re saving.

Does this matter if you're not optimizing for building a ton of wealth to pass onto kids? 

Always keep an eye on interest rates to optimize cash flow and return on equity. if you switch to long term hold investing, you'll want to see if equity stripping is a strategy that works for you. 

A real estate tax pro  can help optimize deductions, ensure compliance, and avoid costly mistakes most importantly so that would be the next step