"Is there merit in a scaled/bracket system similar to our income taxes" Hi , there is a simpler way . Pay the income tax on withdrawals that would have been due if the investment had not been in a protected space.
Also back to an RBL and a reasonable minimum withdrawal amount [ ie an amount that aims to exhaust the super near your expected demise].
That effectively means tax at the marginal rate minus a 15% rebate , for funds accumulated at concessional rates.
To get a handle on how that worked have a look at the old 55 to 60 system.
Tax is then progressive at the same rate, minus rebates , as normal tax.
If you had lots of money in super , you got lots of concessions [ either going in or on earnings] , so taxed more coming out , especially if you have lots outside as well. cheers