At least your period covers both down and up markets.

However, all you’ve shown is that you can fit a curve, with a set of input parameters based on other data. If you have enough input parameters and knobs you can fit anything.

An experiment to test your model is to predict the value from 1982 to 1990 (if you can find the input data) and then post that, without tweaking parameters.

How far forward do you project ? Since you have “Fed funds rate today” as pone of the input parameters does that imply you can’t project forward ?