most people who take on waifus do so to fulfill the need for companionship. blank slates like 'the gardevoir you caught in hoenn' can be molded to be anything you want them to be, hence perfect.
you hold onto your perfect waifu and other females just cannot compare. this, along with her not actually being real, causes depression and grief, as if mourning the loss of a loved one but all the time
given that gardevoirs are reputed at being unquestioningly loyal, protective and caring, your garde-waifu fulfills one of two roles you are missing, a mother or a mate. you say that you want your relationship to be as equals with your voir, which is fine in theory. but its just that.
if you were living in the pokeworld and were a gardephile, you couldn't leave her wild as someone could steal her, Team Rocket or otherwise. So you keep her in a pokeball just once to mark her as yours.
But even if its just once you've still caught her, and all caught pokemon are compelled to obey their trainer. Thus, not equals.
While the gardevoir may be fine with this, and come to expect it, protect and love her master, keep him warm in bed, go on walks, do kinky things with her horn, All of these things she is compelled to do.
Earlier you said that thinking of her in a submissive role was unattractive.
so i can draw 3 conclusions:
1: You truly do want a servant, as illustrated by your desire for a being who is by nature submissive and caring
2: You want an equal, but have picked a creature that is made entirely of your fantasy and as such is subject to your shifting desires, whether or not you realize it or;
3: You still desire that equal and leave her off the collar. In that relationship you'd have to be her perfect mate as well. You desire her, she desires you. reciprocation. But what would an empathic, psychic and barely humanoid creature find attractive in a human man? This is where my succubus theory comes in. (cont)