The equal division requirement ("A uniform ratio [...]") keeps district size below the ceiling unless it’s viable for every state. State populations don't divide into one another evenly without remainder, but requiring equal division means they'll have to be as close as Congress can make them, reinforcing the Apportionment clause. The people can experiment in the future to find a compromise ratio. Unless the people call for another, Congress will probably vote for the largest uniform ratio.
1:600,000 is too large a ratio for effective representation, but smaller states could be shut out at 1:30,000. The range of state populations is huge (see Redistricting Without Gerrymandering). We need a ceiling ratio that will keep the number of people per district within a range one Representative can effectively represent.
By allowing us to go back to Article I's ratio, the clause doesn’t amend
but it adds flexibility. What it stops is the freeze, with its constant reduction of representation and its fudge factors, so amenable to corruption.
Amendment XXVIII, (proposed), Section 3:
"[...] one Representative, who will represent at least thirty thousand
and at most two hundred thousand."
The ceiling ratio, one Representative:200,000 population, balances the need for availability to constituents against the need for manageability in Congress. And the smallest state today would have two districts at the ceiling ratio without requiring at-large Representatives.