The general rule to avoid injuries from falls is - enough substrate to effectively reduce the height and give a cushioned landing. And checking they can't fall more than say 25 to 30cm and land on something hard or pointy. So the sputniks can help with that too as they can hang over the top of a hard toy for example. And you can then tie a tube to the roof of the cage with sisal string - leading from the sputnik to the shelf (or to another shelf eg if the sputnik is next to the shelf) and it makes a kind of extra roof run and also covers up hard items underneath
They can actually monkey bar across the roof and drop the full height and who knows what they get up to when we're asleep.
But for now I would just add some more substrate and think about replacing the hammock and hanging ladder.
The whole hamster toys sale is really annoying - I wasted a lot of money on useless items at first. Hence the list above for tried and tested ones!