Hi man. Thanks for giving my road layout a shot, and I'm glad you had some success with it.
I made a video showing a playthrough with the diagram I posted (although I modified it slightly with one extra, short avenue on the left hand side, due to an intersection issue with the railroad):
This city reached 1 million population in about 3 1/2 hours of play. The video is sped up so it's only 20 minutes (although I'm sure you can just skip through alot of it).
Here are some tips to make the city as big as possible:
To determine the spacing of the roads, use the large tree row under the nature section of the Parks. This is the same size as a high density building. You can see in the video the trick I use to get the correct spacing (although I ended up messing up some of the measurements on the right hand side and had to fix it later when I realized @ 13:09). The spacing between the streets should be the same size as the length of two large tree rows.
You need a support city to provide all power, sewer, water, garbage, fire protection, and city hall functions. Your main city should have none of these. In the video, I placed a city hall for a bit due to a server sync issue, but bulldozed it once the one in the support city was recognized. Only volunteer fire helicopters. Keep fire trucks out of your main city entierely, as they tend to get stuck in traffic and get dispatched instead of the helicopters.
The support city should also provide extra police and ambulance. In the end, mine was sending 100+ patrol cars, as well as several police helicopters, crime prevention vans, ambulance, and wellness centers.
I usually just throw together the support city in about an hour. Get it up to about 75k population with no unemployment or unsatisfied shoppers, then start building services to send to the main city. You can send money to it later to add additional services. You'll need multiple nuclear power plants, water plants, sewer, etc. 1500+ in excess for each will be more than enough. Even if the support city has no money and a $50k an hour deficit, it will keep functioning as normal.
For maximum population you need to keep everyone poor, so don't put things like hospitals and police stations too close together, or put down any parks. You'll always end up with some medium wealth, but the city should be like 98% low wealth.
You don't need schools. They cause too much traffic and take up too much space. With a couple police precients and three or four police stations you can keep crime under control. The fire helicopters will take care of most of the fires.