Dear God, that is the UGLIEST city I've ever seen!
You are obsessed with grids! The road layout just looks like the same identical square pattern repeated for
all eternity. It's actually hilarious that you stopped the grid right at the city limits instead of buying a new tile and expanding further.
Okay, you're obviously a rookie at city-building, so let me give you my first advice:
START AGAIN! This city is a write-off because the infrastructure is a clogged network of squares with no room for expansion. What happens if you want to run a train line through your city? You can't! You didn't leave any space left. Look, here are a three cardinal rules to help you start afresh on a new city:
RULE #1: Spread out your city! Don't just start building from one spot and then jam everything right next to it. You have LOADS of room! Start building a neighbourhood on one area of the map, then move to a different location and start building a new neighbourhood. Then move out further and build a new neighbourhood elsewhere. Leave some space
in between these areas for future expansion like adding a highway or something. You'll need lots of breathing room for a large infrastructure as your city grows, and you obviously have none here. If your districts are very far apart, you can connect them with a simple two-lane highway to reduce travel time and then remove this later as the gap between districts fills.
RULE #2: Don't use grids everywhere! Did I mention this is the ugliest city ever? I'm not kidding. Aesthetics count for a lot in city-building and that includes creating an appealing layout for your blossoming town. Take a look at my city of 50'000:
See that! I didn't rely on grid patterns everywhere. I varied my road layout to keep things diverse, molding it along the terrain. You can also identify various districts and where I left space in between them. If you can't see it, take a look at this screenshot:
This is a newer area of the city. You can see an industrial district on the coast in the bottom-left, a low-density district on the bottom-right, two high-density districts at the top, and a new office district in the center.
Look how much space I left in between them! That's what you should be aiming for. I can fill up these districts as my city grows while still having enough room to weave train lines or large roads between them.
RULE #3: Get a better map! It's hard to tell from one screenshot, but I'm guessing the map you're playing on is as flat as Holland. That's probably not a good idea as it contributes to you falling into a grid pattern without terrain features to hinder you. Look on the Steam workshop and download a map that has a diverse and rugged landscape: Mountain ranges, deep valleys, winding rivers, vast deltas, perilous cliffs, etc. This will force you to mold your road layout according to the terrain and eliminate your grid obsession. Grids are efficient for some high-density areas, but you also want the challenge, fun and beauty of designing a fluid road layout that can fit the landscape.
Sorry if it seems like I ignored your original questions, but your city obviously has bigger problems than just zone density or education. Use medium or large roads where you anticipate larger traffic flow (something that's impossible to do in a homogeneous grid layout). You can zone high-density in new districts if you have enough demand and gradually replace low-density districts over time. However, don't eliminate low-density housing entirely because those attracts families that are more compelled to procreate than the yuppies living in high-density apartments. As for offices, those require educated citizens so you keep an eye on your citizens education to see if your city can support offices. If your city is still young, you should stick with various industries until more citizens graduate.
