Nice starting concept. Have you seen or heard of Parasyte: Maxime? my memory is a bit fuzzy, but it's something you can get a lot of inspiration from. imo the motivations of the "aliens" there made them objectively benign once humans understood their situation, but some would argue otherwise...
you should try capitalizing on the odd mix of scifi (the aliens) and fantasy (the witches) of your story.
the aliens' motivation could be tied into the witch society somehow?
i'm no expert on mental illness, but use that anxiety allegory to your advantage whenever you're stuck. anxiety cannot just magically go away, but we can learn to cope/live with it/contain it. what could average-Joe Finn ever do to overcome this invasive, malicious alien? what if overcoming the alien "taking over" is a state of mind? people tend to feel less anxious when in their comfort zone, and i see he got a gameboy on his shirt + playing a game. what if a temporary solution to "stay in control" is by engaging himself in an activity he's quite fond of? whenever he feels something creeping inside, he pulls out his gaming device. but just like real anxiety, sometimes things work, sometimes it's just so overwhelming to overcome with such simple methods...
I'd advice you to first focus all your thought on Finn and the alien first. list down possible motivations, problems, solution, end goal etc. motivations do not always need to be so grand and complicated: aliens wanna thrive on planet earth? accidental crashing, and finn was their first chance to find a host body? they were looking for something stolen from them? it's actually all the witches' fault?
for each of those potential motivations/problems, list down multiple solutions. the more ridiculous they are, the better! cross out obvious solutions, and give the more ridiculous ones a little thought. you'll find yourself eventually having a clear vision for the direction your story will take : )
again, if you can handle body horror in anime, i highly advise you to give Parasyte a watch! and sorry if my thoughts are all over the place ^^;; i find it a bit hard to properly organize my thoughts haha...
good luck!