IRR Help (non-linear range)

I am building an underwriting model that needs to be “elegant” as it will be distributed to my clients.

I have already solved one issue with calculating the IRR (illustrated below)

=IRR(OFFSET($O$39:$O$399,0,0,R21*12,1))

R21 represents “when” we would choose to exit the investment– in the current iteration of the model it is the 84th month. However, the cash flows are already secured through the 156th month. My issue is that IRR requires linear cash flows, and I need to add the exit price to the IRR range which is located in cell R24, without interrupting the ongoing cash flows to the 156th month (I would rather not create a new column).

Your help is very much appreciated.

Please see the attachment

What you see is a snapshot of a much larger proprietary workbook, that I can't share.
Column B represents the cash flows after debt service (that's the only difference between in-place and net).
B38 represents the equity capital invested (above borrowed dollars).
A34 represents the equity derived from a sale (about loan dollars)
We are employing "worst case scenario) underwriting and that the asset will be sold at a slight net price below the acquisition cost. (This is due to the secured income being derived is above fair market valuation) the sale price is derived by a separate underwriting which marks the cash flows to market

Thank you for catching the 122 - when I originally wrote it, I didn't consider that the capital invested was a "period"