Stratton …
I have an
Ortlieb E-Mate pannier that is designed for daily commuting
and has a special padded enclosure for a spare battery. Sounds perfect but, unfortunately, I cannot comment on whether your Stromer battery will fit. Also, I haven't used the pannier sufficiently to be able to assess durability (but it is an Ortleib!).
Topeak trunk bags feature in numerous photos on the EBR Forum. I like mine.
Be aware that you might encounter difficulties when trying to fit both trunk bag and panniers. This is because trunk bags tend to spread outwards beyond the rack and panniers, perversely, extend above it, each taking over the other's space! This situation can be avoided by choosing a rack with twin horizontal tubes – the panniers hang from the lower tubes; the trunk bag to sits on the upper tubes.
… David