Im no ebike expert but I believe the crack lag problem for hub motors has been mostly fixed by better sensors. There are aftermarket Brompton e-mods that have received favourable reviews with them.

This was the original e-Birdy I believe, https://road.cc/content/news/326-new...lectric-folder. went on sale a while later in the UK. Every so often one shows up on Ebay. The battery bag is a similar solution to the current e-Brompton - albeit the Brompton has the advantage of the bag being much lower down the frame, near the headtube which I imaging improves handling compared to the e-Birdy.