Good luck on your Four Corners run. Since it's been a week and nobody else has replied, here's my two-cents. Maybe this will spark a couple other replies.
1. Yes, gas is considerably more expensive in Canada, there is no getting around it. On the plus side, they price it in Canadian, they measure in liters and they drive in kilometers so you never really know how much it was.
1c One caveat, in Quebec the gas pumps only speak French. Good luck. Most places will take USD if you ask nicely. At worst, if they take it 1:1 you’ve paid about 25% too much. Not great but in a pinch it will work.
2. The road north of Lake Superior is very scenic, it's a lovely run and fairly remote so traffic shouldn't really be a problem. If you've not seen it, or the Mackinac Bridge, they are worth a visit.
2a Instead of making the corner at Toledo you might consider crossing into Ontario on I-69 at Port Huron / Sarnia. It’s an easy crossing and it cuts off all that traffic between Detroit, Toledo and Cleveland.
2b A lot of guys go south of the great lakes and follow Lake Erie north crossing into Canada somewhere around Montreal. Others choose to stay in the US the entire way. On my last 4CX I had to zig-zag a little to finish riding in all 49 states. It depends on what you want to see along the way.
2c A run along the St Lawrence on the US side, while a bit slower, is much more scenic and would let you avoid the 401. But you live in SoCal, how bad could it be?
2d In general, I find smaller crossings are faster than major ones although exceptions occur. Other than trying to cross back into the US on I-5, I've never had to wait very long.
What to pack? In general, less is better. Pack your extra clothes and spares in one bag and leave it locked in your saddlebag overnight unless you need it. Carrying everything into the hotel every night is exhausting.
Pack some light shirts and a wet neck wrap, it’s gonna be hot, Hot, HOT crossing the plains in August. (And wet-vests don’t work east of say Denver.) Bicycle shorts. This ride is a LOT of hours in the saddle and if you’ve never worn bicycle shorts under your riding pants, you need to try it.
Smart phone and / or a laptop. Make up an email list of friends and family and try to post them a note every so often about where you were and what you saw. It will be nice to hear back from them while you’re traveling and, when you get home, most of your trip report will already be written!
Rain Gear. Tire repair kit with an air pump. Minimal tools. You won’t have much time to fix anything serious.
Finally, drinking shoes for Key West. Start with a beer at Sloppy Joe’s, it’s a classic. Then cross the street to the Hog’s Breath for some of their magical rum punch and some good food. That’s my favorite. Eat at the bar if you’re traveling solo, nobody’s really aver alone at the Hog’s Breath. Then maybe a nightcap at Schooner’s Wharf or Captain Tony’s. By then you will have earned it! Cheers!