Road bike recommendations

..... when we don't change the effective top tube length......

..... with a fixed top-tube.....

Stop fixing the ETT. It needs to change with the STA.

The argument is that reach is STA independant. But you need to be comparing frames of the same reach, and varying ETT, and STA as needed to illustrate this. As soon as you only vary one. Its no longer a relavent example.