Credit unions are good for car loans. I'd explore those options before loaning against retirement funds.
This is why I fund a whole life policy with maximum accumulation of cash value. Basically I put in about $400 a month. My son started with a $185,000 dollar untaxed death benefit if something happens to me. After 3 years of funding it I have about $10,800 in cash value built up and the death benefit has increased to $219,000.
The concept is known as infinite banking, basically if I need 10k or less for anything I would pull it out as a loan from my policy and pay myself back at current interest rates instead of the bank. By the time I'm 50 i'll have a guaranteed cash value of $134,000 and guaranteed dead benefit of $425,000. If I owe on any loans when I die they just take it out of the death benefit.
This concept is exactly how the rich get richer and in another 10 years I won't have to fill out a loan application again in my life. I also have the option of overfunding up to $10k each year if I want to build the cash faster.
At age 75 I'll have 379k guaranteed cash value and 565k death benefit. I can either use the money in retirement as a loan and not have to pay taxes on it and then my heirs can get the difference untaxable as well.
Dentist, have you ever considered something like this to set up payment plans for your valued customers that you can personally finance and then collect the interest yourself?
ETA: I bought the policy when I was 29. Into year 3 right now.
ETA: My guaranteed accumulation is right at 3.9+%. Good luck finding that anywhere else, especially with the ability to plug more in. PM me for details on the company I used.