“And the happiness that we will achieve by getting rid of our desires is as good as the happiness we would get by fulfilling them”
It seems that Epicurus sees happiness as being a state when it seems to me it can also be a property of a process (or transition between states if you like). Have you ever noticed that food smells and tastes better when you are hungry? That eating, the process of satisfying your hunger, or transitioning between the state of hunger and the state of satiety, is more pleasurable when you are more hungry? And also more pleasurable than the state of “not hungry”.
I’m sure you can find any amount of modern psychological research showing that there is a spike of pleasure in acquiring a new car (or buying a new house, or getting a raise) but mostly that spike fades and your level of happiness recedes to your baseline. Interestingly, this also seems to apply in reverse: even following as large a trauma as losing a limb, people’s happiness tends to revert to their baseline from before the trauma. (https://en.wikipedia.org/wiki/Hedonic_treadmill)
In short, it seems an oversimplification to see happiness (solely) as an attribute of a static state rather than (also) as an attribute of a dynamic process.