[Nagí]

So I keep search and I found something very interesting and it gives me an idea.
It's an AddOn called EnhancedClassIconPortraits, and this is his core:

local TEXTURE_NAME = "Interface\\AddOns\\EnhancedClassIconPortraits\\Textures\\%s.tga"
 
hooksecurefunc("UnitFramePortrait_Update", function(self)
    if self.portrait then
        if UnitIsPlayer(self.unit) then
            local _, class = UnitClass(self.unit)
            if class then
                self.portrait:SetTexture(TEXTURE_NAME:format(class))
            else
                --[===[@alpha@
                print(("EnhancedClassIconPortraits Error: UnitClass returned nil for unit %q"):format(self.unit))
                --@end-alpha@]===]
            end
        end
    end
end)

If I take a similar way as above, I can avoid to call SetMask() and have the same result without performance issue. So I have extracted the textures and applied directly a mask on it. Now I have a folder with 46 icons, here is an example. Maybe the name of the textures in the folder could need to be renamed if I want to call them.

That's probably less efficient than SetAtlas(). What do you think about it? Is that possible?