Assalam!
If someone told me they were overthinking about death, I would suggest them to switch from thinking of "death" to thinking of "the Hereafter". Because DEATH itself it simply a transportation to the Afterlife (May Allah protect us all from His Wrath and any punishments of this life & the Hereafter ameen).
So THEN I would answer - I don't think there's anything wrong with "overthinking" about death (if thinking of death = Day of Judgment/Afterlife, astaghfirallah) because then one will watch every step he does while seeking the forgiveness of Allah by reciting "astaghfirallah" all throughout the days.
With that being said, STILL a Muslim should think positively and optimistically about all aspects in regards to Allah. So when thinking of the Day of Judgement, understanding initially that that Day is a frightening Day for those who are wrongdoers, we have to be hopeful of Allah's Mercy and think of our blessed, easy steps towards Jannah (may Allah protect us so we are not amongst the wrongdoers, ameen and may Allah make us among the Righteous who run towards Jannah happily and eagerly, ameen)
My thoughts! Love everyone, Salam!
-Faraz from "FarTaha Theory"
https://linktr.ee/FarTaha_Theory