Better not to save on batteries - or rather not get an ebike at all. The original parts are comically expensive but necessary. Also storing these things within apartments might not be the best idea. Charging while not there even less. Or use after shock. (E.g. after falling to the ground) FWIW there are charging bags but not all can stand the pressure of a thermal runaway (-> see YouTube videos)

Another option is to wait a few years until LFP batteries will become officially available for ebikes