I think you're right and it is interesting observing their behaviour! They do love houses that are dark inside and she probably loves the Rodipet house. She'll have less to do in terms of sorting her toilet corner etc, so you could add other interests to get her out and about more. Although I think sleeping more and staying in the house more is very much a winter thing - our Syrian isn't out as much either, except at feeding time if I'm late when he sits on his shelf waiting for the piece of veg to appear!
Is it a detolf she is in? She probably uses all of the space at night during their most active time, but as it's a lot more space than she had before she may feel a bit exposed in the cage. With a large cage it's important to have plenty of overhead cover. The Germans often fit a long shelf along the back, the whole length of the tank - the hamsters can then go from one end to the other under cover if they feel a bit exposed.
Another option is to fill up the floor space with large toys and tunnels but keep one end with very deep substrate for digging in, which effectively reduces the amount of the rest of the floorspace so the amount of toys doesn't get excessively expensive!
I think large cork logs work really well in large cages. Have two in our 100cm Barney. The medium sized Rodipet ones are good. They are light so don't sink in the substrate (in case they're tunnelled under) - they make a nice large dark tunnel for travelling about the cage, and also make an interesting thing to climb over with an interesting texture so the cage floor is less flat and more of a climbing obstacle course in places!
I do tend to stick the cork logs in a plastic bag in the freezer for a week though, just in case there's anything living in them! They often have bits of hard moss etc stuck to them but it's not harmful and none of our hamsters has ever chewed it anyway. I also bought a bag of cork pieces (about 5" square) which you can lean against a cork log to make a hide or another ramp, or stack them like steps.
So I'd go with 2 or 3 large cork logs or tunnels on the floorspace, a different level of substrate at one end of the cage, and if you can, a long narrow shelf along the back. Just a piece of plain pine wood with dowel legs glued on is all that's needed. Although if it's a detolf it could maybe be half the length.
These might give some inspiration
https://www.flickr.com/photos/125312...6/14203399270/
This one is for the shelf idea - not good with no lid!
https://animalhomes.tumblr.com/post/...e-hamster-cage
They do need a platform somewhere really in a large cage - something to sit under and climb on.
Apologies if you know all this already! Winter does make a big difference though, as does room temperature. Ideally the room would be at 20 degrees all the time - if it drops below that they do tend to snuggle in bed. I even have a little oil filled heater on a timer that comes on for an hour or two during the night.