This is where I disagree because I don't need to consider ISO when setting the exposure*.
Basically as I see it, there are two broad categories of photographers in the way they set exposure*.
Those that set ISO first and then aperture and/or shutter speed to suit.
Those that set aperture and shutter speed first and then set ISO either manually or automatically to suit the exposure* they have set.
I am in Group 2.
You also haven't defined what you mean by 'optimal exposure' as you use it in your post. For me, the optimal exposure** for a given scene can be significantly different if the aim is a nice looking sooc jpeg or if the aim is to maximise the quality of the raw data in terms of maximising the Signal to Noise Ratio.
Group 1 photographers are more likely to believe that ISO is an exposure setting. By setting ISO first they are locking in a predetermined exposure* by biasing the camera's meter to zero/center on the exposure* for that ISO setting which might or might not be optimal for their output as you correctly mentioned.
Group 2 photographers are more likely to be aware that ISO is not actually an exposure* setting and so will more than likely set the widest aperture (smallest f-number) that will give the desired DOF and the slowest shutter speed that will meet motion blur requirements for the final image. They will then either manually or via Auto ISO set ISO to output an image lightness, as long as highlights are not clipped, appropriate for the exposure* they had already set. If at base ISO there is highlight clipping then obviously you need to compromise on DOF or motion blur depending on which is least important. The final image lightness is then set in post.
* exposure - amount of light that struck the sensor per unit area while the shutter was open
** optimal exposure - the maximum exposure* within dof and motion blur requirements without clipping important highlights.
*** under exposed - more exposure* could have been added with the DOF and blur constraints still being met without clipping important highlights.
I don't believe we disagree. "Exposure" and "optimal data" are NOT the same thing.
The exposure at sensor is what it is from how it got there (light->aperture->shutter speed). If the ISO is set too low, you do not have optimal data because you're stuffing all the exposure into too few bits. If the ISO is set too high, you do not have optimal data because you've truncated the data. Your Group 2 is simply trying to get optimal data from the exposure.
Back when Ansel's books (including the Time/Life books) were the primary source of real information, the consistent use of terminology and constructs got everyone (except for perhaps Kodak ;~) thinking the same way. In the digital era, we have NOT had the same consistent use of terminology and constructs, and worse still, we had (have) a lot of mythinformation (wording intentional) that gets repeated and repeated. Even dpreview was late to the game on a lot of this stuff, and only got involved with it because the fora commenters were trying to whack down incorrect responses. Iliah was particularly well known for trying to use the Socratic method on errant posters, but it apparently was all greek to them.