You'll have about 5k a year in principal paydown. 

Not all things you consider an expense are honored by the court.For example, in a Colorado Court of Appeals case, “the trial court found that the principal portion of the mortgage payments did not qualify as ordinary and necessary expenses for purposes of calculating child support.”It’s All In The NumbersAccording to the Census Bureau, “Parents who received regular child support payments received a monthly average of $604 and a monthly median of $396 in 2017.”

Instead of paying down a little principal for a little time, pay down a lot of principal and refinance to a loan that allows a lot of time.

Its the closest thing to free money there has ever been in our history.On 390k and 3%, your principal paydown is probably 700 to 800 a month. 

Hi Samuel,When considering how to pay off a loan faster, both methods you've mentioned—making a large principal payment at the end of each year versus bi-weekly payments—can be effective, but they operate differently.Large Principal Payments: Making a large payment once a year can significantly reduce your principal, leading to lower interest costs over the life of the loan.

With higher price points, you have bigger principal buy down.4.

I'll assume no principal pay down and simplify by assuming the next property will also cost $400,000.

Not sure what the difference would be $ wise  but reducing your principal biweekly vs at the end of the year would reduce your interest expense more than lump sum. 

Here's a https://www.irs.gov/businesses/cost-segregation-audit-technique-guide-chapter-4-principal-elements-of-a-quality-cost-segregation-study-and-report to the IRS website noting specific items that are included in the cost segregation study report.

Locking in such a low rate means more cash flow if you rent it out, even with your HOA and property management costs factored in.Let’s tackle this using your numbers:Current Rent: $2,950/monthAnnual Tax, Insurance, HOA: $7,000Mortgage (principal + interest): Roughly $20,832/yearWithout even factoring in maintenance or vacancy costs, you’re likely cash flow positive.