About the proprietary battery, I was meaning bike brand proprietary battery and in this case proprietary for one bike model vs. mid drive motor brand proprietary batteries like Shimano, Bosch, Mahle... batteries that are used by many bike brands and available outside of the bike brand resellers network, are produced in much bigger quantities and spare batteries availability is guaranteed for a long time.

With a battery proprietary to one bike model of one bike brand, this battery will most probably only be produced for the bikes produced with very little spare batteries available.

You are right about the gear inch range of current Brompton Electric, the lowest gear inch is not short enough meaning that for steep hill climbing the bike is unusable without e-assist.

The good news is that there are now third parties components to easily to upgrade the Brompton 4s gear system to a 7s gear system with a 11-32t cassette that gives about 23 gear inch with a 44t chainring (but its a pity hat Brompton doesn't propose it on their bikes).