Well, what I really wanted was for my city center to be located at the tip of the peninsula, but that required me to sprawl out and grow my population enough to purchase two tiles in that direction (plus one in the opposite direction for rail access). I was trying to plan several hours ahead while sticking to the vanilla rules, rather than doing infinite money / 81 tiles.

At 79K population, it now looks like this:

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I guess it would be more ideal for the highway to run alongside the river, but the system interchange (the cloverleaf) and actual highway was outside my city limits for a while. Basically the highway runs through the center off the sprawl, then becomes a tunnel underneath the downtown residential area, connecting a couple blocks apart to the main avenue which borders the commercial zone. Normally a bad idea, but since it is a peninsula, there is nowhere else for traffic to go. The highway just terminates in the middle of town and it works. Anyone proceeding though the tunnel is trying to get downtown anyway. There is no thru traffic coming out getting stuck in commercial / residential neighborhoods.

Here are some more pictures.

Downtown:

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Transit Hub:

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Marina:

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The amusement park is an absolute shit show, despite the fact that there are two separate metro lines, intercity rail, tram, and bus service within two blocks. The shitshow is very localized to the front gate though. Just hundreds of tourists pulling out their pocket cars. Doesn't really back anything up though, it is all traffic leaving the park, and it disperses without any issues.

The city is beginning to suffer an economic crisis though. Everyone is too educated and a lot of factories are going out of business, leading to a demand for imports which is impossible to fulfill. The cargo harbor/train hub (put in after these screenshots) never stops pumping out delivery vehicles. These vehicles are perfectly capable of navigating the city and returning to the harbor without getting stuck, but it takes a tiny bit longer for the empty trucks to enter the cargo hub than it does for them to leave. This creates a queue which never stops growing until the garbage trucks get consumed by the blob, people get sick, the ambulances get consumed by the blob, people die, and then the hearses get consumed by the blob. This has forced me to turn off TM:PE's "hard mode" traffic, which prevents despawning, but I had it enabled up to this point.

At this point I'm doing a lot of fiddling around and re-loading to see if I can come up with a solution which still works on hard mode and doesn't require using the bulldozer as an instrument of economic policy.