With my current setup, would installing PulseAudio make matters better.
Some time ago I did some tests about the relative volume on speakers and headphones with the plug-in switching .... but cannot recall if I ever got a definitive answer.
Instead I just stopped using the plug in headphones and use USB wireless or bluetooth devices which are treated completely separately and have their own controls in PulseAudio Volume Control.
I *think* I was of the view that the switching was happening, but that there should be more user control, and that this just might happen in the future.
For you own case I suggest trying PA for about a week.
It might help or you might dislike it ..... but you would need to give it some time to see if you can get used to a different method of applying your preferences.
PA is most definitely for me .... I have multiple sound devices and want full control over them and what sound stream is sent to them. Having them all plugged in at the one time makes no difference to my control, as they are all treated separately ---- with the exception, as I recall it, of the plug in headphones which now switch off the speakers