How to improve the loop closure module?

[PrePLEX23]

I am trying to make some change to improve the performance of loop closure module.
Inspired by LIO-SAM, I registrate the new keyframe with the submap composed of candidate keyframes nearby. And the constraint is only relative to new keyframe and the center keyframe of submap. But the result show the implement may be incorrect.
I think maybe the information matrix should also computed using the submap?
Any advices will be appreciated! Thank you.