I know that feeling
My first (well second) cage was the same size as the Hamster Heaven and it seemed massive. But Charlie also grew massive and needed a big wheel and big toys and suddenly it was cramped! I think a well set up Mamble like yours is a brilliant environment - lots of things to do. I upgraded Charlie to a 100cm Barney earlier this year, aware that he was not young any more, so his set up was good for a while but he is now an old guy and has everything at ground level. I just wish I'd given him a bigger cage earlier!
The sisal string can be a bit of a fiddle but it's ok when you get the hang of it. I'm not sure how easy it would be to tie that tube to the bars as they are bendy and can move a bit but I'm sure you'll work out a way to attach it securely!