Re-implementing one's own proprietary code as open-source
I am not a lawyer, and if you are seriously considering going ahead with this project, I suggest consulting one.

The problem is that copyright infringement is not limited to direct copying. See, for example, the "My Sweet Lord" case. George Harrison is not believed to have consciously copied "He's So Fine", but he had heard it.

You have much deeper familiarity with the existing code. It is going to be very difficult for you to write anything similar without being inspired by what you wrote before.

If your former employer's reluctance to release is related to liability and support issues, they may be willing to permit re-implementation, so it is worth asking. If they are hoping to make money off the code, they will not agree.