With the fact that you don't have enough data points to determine supply vs. demand and how it equates to value of the basement, I think you'll have a hard time getting him to budge or proving your case.

Look at the data below. 

Then compare that data against similar rentals in the market.

While I don't have data to support, I have a hunch there are a number of smaller landlords which incorporate and are a "corporation" for purposes of liability.

Try not to touch those other assets if you can help it, especially the IRA and stock markets as those will likely increase in value in a few months so selling those isn't wise right now.

Would really like to move forward, but not if it is going to crash in value, or rents go down too much?

If you are actually considering writing an offer you need to get actual data.

Your returns in value should be 5x+ and a nice rent bump if a rentalDid you change the ceiling? 

Both are multi-family deals and if the market actually takes 15-20% correction, it will be significant cut in value.

I have great relationships with a couple lenders where we live and it is invaluable.