Interestingly, almost all of this growth seems to be coming from users upgrading from April 2018 Update (1803) as October 18 Update (1809) is hovering around 30% steadily.

That should come as no surprise. Microsoft has made the 1903 Feature Update an entirely optional 'Download and install now' across the board (including for Home).

...based on our data it looks like the rollout is still very cautious and linear.

Microsoft can no longer set the pace of the rollout. They can (and doubtless do) offer it to as many users as possible, but they won't force an upgrade on a 1809 user until near end of support (May 2020). The 'caution' being noted above now entirely reflects that of the users.