[Classic] Toggled spells: stance bar, action bars, and :SetTexture

[Tair]

Hi everyone.

I enjoy writing small addons that are by no means practical but address personal visual gripes and help me learn a bit about the addon API in the process.

In this instance, I dislike how toggled spells (Devotion Aura, druid forms, hunter Aspects, shadow form) have their icons all change to the same icon whenever they are active, which seems to be: Interface\Icons\Spell_Nature_WispSplode

My goal is to prevent the icon from changing and simply desaturate it. I think the ideal approach would be to intercept the function that changes these icons to begin with, but looking through the default interface code, I haven't determined if that's possible.

In lieu of that, I've temporarily narrowed my scope to the stance bar and have come up with a method that fires on UNIT_SPELLCAST_SUCCEEDED to desaturate the icon, but I haven't successfully reverted the actual icon texture.

for i = 1, 10 do
        local btn = _G["StanceButton" .. i]
        if not btn then return end
        local icon, isActive, isCastable, spellID = GetShapeshiftFormInfo(btn:GetID())
        local iconTexture = _G[btn:GetName() .. "Icon"] -- the button's icon
        local texture = GetSpellTexture(spellID) -- the texture ID/path
        if isActive then
            iconTexture:SetTexture(texture) -- doesn't change the texture; is this because by the time this code runs, the icon has already changed?
            iconTexture:SetDesaturated(true) -- works 
            iconTexture:Show() -- force the icon to update?
        else
            iconTexture:SetDesaturated(false) -- works 
            iconTexture:SetTexture(texture)
        end
    end

In terms of the default interface code for the stance bar, I believe the icon switching might be occurring in the function StanceBarMixin:UpdateState():

if ( isActive ) then
	self.lastSelected = button:GetID();
	button:SetChecked(true); -- it's most likely this that is changing the icon
else
	button:SetChecked(false);
end

However, based on the searching I've done through the code so far, SetChecked() seems to be a fairly common function and thus far I haven't found any specific code that where this function is switching the icon.

I am by no means an experienced addon developer, so I would be grateful for any perspectives here. Is this even feasible?