To prevent IRS recharacterization as a gift (or as OID in some cases) you should charge at least the relevant AFR for the month in which the loan is made. Make sure the loan provides for compounding interest at the points set forth in the chosen AFR.
Here's a website of a lawfirm in SF that publishes them regularly:
For a 30 year secured loan, you can charge the long-term AFR compunded annually. This month that rate is 4.35%. It will go up to 4.4% if you don't close before the end of March.
This does no necessarily mean it is the rate a fiduciary should charge. That might be higher.