I’m trying to make additional money to speed up purchasing an investment property

With that loan product, they will not be looking at your DTI or work experience and will qualify the property based on the DSCR (Debt Service Coverage Ratio), which is a measure of the property's cash flow.

If you don't have any debt, a 750+ credit score, and income, you should not have any issues being approved for a mortgage. 

Is there any advice you'd give me on our particular situation:Husband and I are 50-something "millionaire next door" types with no debts and significant savings.

It depends on the type of loan, loan to value, fico, income, debt, and experience. 

She is in between david ramsey (no debt ever) and Rich Dad, Poor Dad (super leveraged).

All paid off and very little CC debt.

The interest payments will be additional deductions/expenses. 

Don't submit the bank statements to underwriting unless you actually need them to close the deal however, as it can cause additional steps in the process for you and your client (if an underwriter sees money moving around, it can cause you to have to paper-trail the funds back to their source, which often isn't a lot of fun).If you haven't completed a loan application, start there.   

In addition, the amount of insurance is limited to the amount of the debt outstanding.