> While Bridgetown has diverged somewhat from Jekyll in terms of architecture and is not source compatible (for example plugins and themes)

Based only on what I've heard, but my understanding is it's not a huge undertaking to port over

This is a pretty important question. The themes are the main reason I continue to use Jekyll

Only with a few modifications …we're nearly through a stint of addressing tech debt, and then we plan to publish a guide for porting over Jekyll themes out for to make this process much more clear.