Biden should have simply refused to run again in 2024. There was already signalling to this effect in 2019, though it was never explicitly promised.[1][2] If he had just kept to that, there would have been plenty of time to run a primary and an actually strong candidate could have been selected.
My understanding is that Biden was very concerned about his legacy and there are rumors about the "no daylight" phrase he spoke with Harris on her campaign and probably part of the reason she lost. But Biden should have dropped down much earlier and will taint his legacy forever. At least he passed the infrastructure bill.