Never said the feasts were established by a people.
"And where exactly did God establish the feast of Hanukkah?"
Where? In Jerusalem at the temple in the 2nd century BC at the time of the Maccabean Revolt. When 1 days worth of oil burned for 8 days for the re-dedication of the temple after it had been defiled by Antiochus Epiphanes. This represents the 're-dedication' of the human body as the temple of the Holy Spirit in Christ as his conception occurred at this time. Hanukkah is known as the Festival of Lights and Jesus said that He was the Light of the World.
In the NT, this is referred to in John 10, when Christ went up to the temple at the time of the 'winter feast of dedication'. It was when the Jews demanded that Christ 'tell them plainly' whether He was the Messiah. After which, they took up stones to stone him for blasphemy.
"Kinda like Christians celebrating Ishtar instead of ‘Passover’!"
Nope. Hanukkah was thoroughly YHWH-centered, unlike Ishtar which is pagan throughout. The mistake the church has made throughout history is to try to incorporate pagan (meaning nature-worship) 'holidays' to make it easier for pagans to accept Christ. It would be much better to teach the Jewish feasts and how they prophetically point to Christ rather than to continue making the same pagan error.
"Mark your door posts so the ‘death’ angel would ‘pass over’."
Yes, the 'blood of the Lamb' saves from spiritual death. That was the central prophetic message of the Passover.