Creaks can be tricky on any bike but I'd work my way from cheap to expensive which it sounds like you're already doing.
- Pedal bearings (sounds dumb but there's nothing worse than spending hours on stripping a bike and finding that it's just a bum pedal!).
- Crank arms - check the torque and, for peace of mind remove, grease, and refit (ensuring correct torque).
Since you've done these then it's tike for a bit more in depth work. The next two I usually do simultaneously:
- Motor housing - see above, grit and grime can get in and cause noise. Remove, clean, refit, and potentially a touch of grease on contact points.
- Motor mounts - this is 99% of the time the culprit of the creaking occurs on both sides of the bike and can be replicated with the bike static. As with everything else: remove, clean, grease and and all contact points and replace, with correct torque. I've done lots of Bosch equipped bikes with similar symptoms as yours which this resolves.
If this doesn't sort it then it's likely internal: bearings, etc. some of these are sometimes user serviceable and they're the typical culrprits, but worst case it's bike shop time...
Edit! Just a quick post-script to this now I've had a few coffees - often I've had experiences where motor bolts are just a little under torque, so it's always worth a quick nip up to spec before you start taking stuff out!