Is this a mistake? I know the cons regarding opportunity cost and after tax repayment but thinking it may make sense in our situation.
It's not a mistake, but as you say, there are significant disadvantages to borrowing from a 403b, like the opportunity cost of the $20,000 not bringing home investment returns, and having to pay back the 403b loan with after-tax income, with interest. As an example, the rate to borrow from my 403b is 4.25%, a rate that will likely increase in the years to come.
It makes more sense to borrow from a 403b if reducing your adjusted gross income brings you access to benefits besides reducing your tax bill. For example, reduced student loan payments (some make payments through IBR, 10% of their AGI), increased health care subsidies, etc. If there's no benefit besides reducing your tax bill, the overall advantage of borrowing from a 403b to do home renovations becomes minor as compared to just saving up the money over time. And there's the mental stress of keeping up with payments on a $20,000 loan.
Another option is to take out a home equity line of credit to do home renovations, if you own your home.