SMB-West provided fuzzy window covers for the front, side, and rear windows with our 2017 Sprinter, plus a snap-on "privacy" curtain between the cab and the cargo area. We only use the latter in freezing weather, to reduce the volume that needs to be heated, and it works well for that. Anything else that reduces volume and/or buffers the daytime heat (close all cabinets, fill up cargo space, use lidded cargo boxes, etc) can help a bit. I guess SMB switched to Arctic window panels recently, does that mean you don't have any sort of partition?
Note that the Espar's power output, as installed by SMB in a Rixen
comfort system, is less than a third when run in electric-only mode. You'll need to burn diesel to stay warm in super-cold weather, even if plugged in. And of course, make sure the Espar is in good shape -- check coolant levels, maybe get an inspection.
That said, I also sleep in the back, width-wise on a raised platform right against the rear doors, and totally agree that a lot of cold seeps in through the doors and curtained windows, even with the "extreme" insulation package. At the moment, I just shove an extra blanket up against the door, but hope that some better ideas will emerge on this thread!