The OP is asking for flexible beatgrids and also a way to shrink and expand the grids to make bpm changes.
This has been requested by numerous folks
https://community.enginedj.com/t/variable-grid-bpm-grid-realignment/15192
@Jesterrip this feature is not available in Engine Prime yet.
Although you can “warp” in engine prime ie grid the tracks to a fixed BPM but it’s hard work.
Go to the grid settings and use set a new downbeat whenever the track starts drifting. You will have to do it a number of times depending on the tempo drifts.