I find you are not alone in thinking after physical death that eternal life is instant, or nearly instant.
There was No instant eternal life for the believing friend of Jesus at John chapter 11.
There was No instant eternal life for believing John the Baptist at Matthew 11:11.
Even dead Jesus was resurrected ( Not instant ) but days later.
Gospel writer Luke wrote that eternal life was Not instant for King David at Acts of the Apostles 2:34.
Luke also penned for us at Acts of the Apostles 24:15 that resurrection is Not instant but future.
There it informs us that ' there is going to be ' (future tense) a resurrection.....
At around the coming time (Not much earlier) of Matthew 25:31-33,37,40 the ones of 1 Corinthians 15:51-53 have that first or earlier resurrection as per Revelation 20:6. That is because they are the ones chosen to be heavenly kings and priests over Earth as mentioned at Revelation 5:9-10. 2:10. They are like the ones of Luke 22:28-30.
They are the first fruits of the sleeping dead ( Not instant dead ) according to 1 Corinthians 15:23 B.
Please notice part B because it lets us know the time frame when the first-fruit resurrection takes place.
The dead humble meek people who will inherit the Earth are Not part of that first resurrection, but later.
John the Baptist knew Jesus as Savior but John is Not in heaven according to Matthew 11:11.
Jesus' friend in John chapter 11 knew Jesus as Savior, but Jesus did Not resurrect his friend to Heaven.
His friend was dead for about 4 days before Jesus gave him a physical resurrection back to life on Earth.
Since Jesus tells us his dead friend was in a sleep-like state at John 11:11-14 then there was No instant eternal life for his friend either in Heaven or on Earth.
So, to me there are two (2) resurrections: Only some believers to Heaven, but most believers to be here on Earth.