As a side note: Getting your industry/commercial balance right won't help with your traffic problem. It doesn't matter if the trucks are coming from your own buildings or from outside the city they will still be traveling through.
Of course it will matter, if you are generating lots of freight it will increase traffic. If you reduce traffic is will take vehicles off the road. Not sure why you don't think that is true.
Here's a video I thought the game had issues with with exporting, When I only have it out of balance. Notice the heavy traffic:
Here is the same exact city where I reduce industry to semi-even it out. Notice the tremendous difference!
You need to step up your traffic game and specific advice on that subject would probably require pictures from you. However I can deduce based on the most common problem, you have a bottleneck. Make more access points to the highway in your industry area so all the elevated traffic that industry generates will spread out more instead of all trying to use the same access point.
This is also untrue. You can see in my city that I have a single 2-lane, 2-way road in and out of my city. All traffic flows fine without making extra on/off ramps. If your district becomes too large, then you may run into trouble and overload the entrance as you can't balance 100%, as you already mentioned.
Also, on the subject of balancing districts, there is a limit to how far deliveries will take place before the game will export your product and import what the destination wants. Imports don't seem to time out and will keep trudging through which will cause traffic to rise from being on the road so long. but you can reduce it to almost nothing.
He's a video of my Snowfall map when it first came out. I lost my old video, it isn't on YouTube anymore, so I made a new one. It seems the game is a little more optimized on these last few updates for freight as it only timed out when snowing, where before it got worse when snowing and timed out all of the time until I rezoned the rear of the map.
Remember, this game only has a 16k traffic limit. So it has to despawn traffic, or more so, not spawn it at all when balanced. Otherwise, our 100k city would collapse and we'd never make it past 9-tiles, let alone filling 9-tiles.