If Aetherpact was a pet action instead of a player action, it would make the usability much better. This would allow you to order the faeirie to do fey union while you're in the middle of a cast. The range also needs to be increased, its pitiful short. As a result the faerie has to move into range first before it begins the tether, which is ~2s delay.
The potency also needs to be increased. You can get your fey union tics up to ~8500, but that will likely get patched back to 5500, which really doesn't feel impactful enough.
Allowing actions such as convalescence, rouse, mantra, illumination to actually increase the potency of fey union would really help as well.