The law does apply to obligations incurred by sole proprietorships.
And, under the holding in the Cathey case, it would also apply any time the military person is a co-obligor on the debt.
There are also provisions relating to guarantors that you should be aware of, that are detailed in my other articles on this.
If the loan is a single payment loan with monthly interest payments, I would recommend applying the reduced interest rate to reduce the monthly payments.