Will-based morale rules are flawed because they conflate how brave you are in the face of danger with how inclined you are to stay in a fight. Moreover, just because you retreat doesn’t mean you have panic and fled. I’ve had enemies make a fighting retreat when they failed their morale check because that was just appropriate to what they do.Yes, the GM improvising Will saves when enemies are bloodied or lose numerical advantage etc is easy.
I’ve recently started to consider a mildly Will-based alternative. Something like make a Will saving throw versus your Wisdom DC (with modifiers for how committed you are to the fight), but I haven’t gotten to the point of mathing it out to do what I want. My current system is just using morale from B/X unaltered (link is to OSE, but I was inspired by this post on the Alexandrian). I decide the creatures’ morale scores during the fight based on how committed I think they are to it.