So, in-game cheats are generally not allowed for a TAS... We can still hit 600k without the debug mode, anyway.
I saw the simple layout that hits the high population. It was made to be simple. It doesn't take advantage of the fact that industrial zones only needs a single rail to work. Along with commercial zones, police departments, and fire departments, but that's less important when there aren't as many of those around in the layout. But it is still a simple layout, the simplest possible that still meets the goal of 600k+ sims.
As a test, I built the layout myself. Here's the history:
1900 JAN:New mayor elected
1900 FEB:Became town
1900 JUN:Became city
1901 FEB:Became capital
1901 MAR:Became metropolis
1901 AUG:A plane crashed! (Happened right by the Your House...)
1902 MAY:Became megalopolis
1903 JUL:Population came to 600,000
That was an unassisted run by emulation, but we can already freeze time anyway. Money glitch was used, of course!
Your transportation can be broken up in lengths of 1 to 3 tiles, and still cover all transit-related demands. Residential zones are the only ones that need to go someplace (
any building that isn't R-zone or nuclear power plant), requiring at least 2 tiles, but everything else is perfectly fine with a single tile of rail leading nowhere.
Crime can run rampant in the city, as it only affects land value upon hitting a certain value.
Sims can complain like crazy how you're doing a bad job, yet they come flooding your place anyway.
Screw sensible planning!
Then again, a lot of it will be waiting and doing nothing. Setting up the whole thing might actually be more entertaining than the actual passing of time for them to grow. I say the priority should be to make the most senseless-looking arrangement and still get a 600k+ megalopolis freaking fast.