I hear you on spending more $$. I really don't think a shelf is the same as a few perches though. A shelf is pretty much the same as your cage, but cut in half or quartered. Wallace is probably a 7 footer give or take and he's always perched at night and in the mornings. He can hang down between the perches, wrap around them or zig zag between them. The dwarf/SD populations are said to be a bit more arboreal. Mounting perches won't be an issue and a few holes in plastic won't ruin your cage.
Your girl doesn't seem to be overfed so I'd expect her to be active. Many folks have luck keeping them in small cages with no stimulation. I guess that's up to the keeper. I prefer more stimulation over just throwing rabbits into a cage and then seeing my snake curled up in the same position every day. Your girl is at a wonderful size if she tops off. I guess the gamble with any retic is the potential for extremes.
Another option you could try that doesn't require cage modification is to let her climb on a ladder when you have her out.
I don't remember the dimensions of your cage. Is it under 2 feet tall?
Try the ladder trick if you have one and see if she will climb around and get some energy out. I really want to see her do well. Also, out of curiosity, is SHE 100% a she? I would think so based on size, but maybe if she is a he, Gerald might be causing a bother.
Keep us posted, I know you'll get it figured out.