The big shelves sound good
As you say there's always a gap (and they always find it lol). If he's such a chewer, hammocks could be a bit of a problem but Zooplus sell grass hammocks for about £3 and someone on here used a chill n chew mat as a hammock as well. You'd still need to keep an eye on them so they don't fall down if chewed I guess.
It sounds similar to setting up a Mamble, even though taller. Another option to deal with some of the height could be to put cardboard round the inside of the bars, just above the base - maybe 6" high cardboard all round - and have much deeper substrate. More cushioning but also reduces the height by about another 15cm.
Putting a full level in could solve most of the issues - if you measure the inside width and depth a hardward store could cut a piece of wood to size and also cut out a section maybe 3 to 4" by half the length for an access ladder. Then you'd just need a shelf over the access part and the corner shelves add additional interest and levels as well. The full level is probably the safest way to prevent falls for a chewer.
Anyway as CMB says maybe post a photo and we can spot areas and make suggestions.
If it was me I would go with - full level plus an extra 6" of substrate
Plus your corner shelves on the upper level.
Then you'd end up with something similar to this
https://www.amazon.de/Schulte-M%C3%A...los+k%C3%A4fig
The Carlos cage is 89cm tall - the equivalent of two Barney cages on top of each other. Instead of the extra cage base half way up you'd have a wood level - so ideally it would need to be higher up than half way as it's harder to land on than substrate. To avoid having a very steep ladder up from the base, the extra 6" of substrate with the cardboard round the base, would mean a ladder/tube access wouldn't be so steep.