Quite refreshing to see you have done your research, so at least have a clue how things work here. As you've found, getting a bike registered should be no problem. Your chosen areas and routes are good too!
I've helped TWO sets of foreign riders buy & register bikes ... two Kiwis bought KLR's here in California, rode to Tierra del Fuego and sold them off. An Irishman and German also bought bikes here. All 4 bikes registered in California and legal. (I mailed the permanent titles onto them in Mexico)
The Kiwis (on our advice) arrived in USA with NO riding gear since it's MUCH CHEAPER HERE than NZ. So while we shopped for bikes, they ordered everything they needed online ... they said it was 50% to 75% cheaper here than New Zealand.
Not sure about prices in Oz?
If you want to try gear on before you buy, (I would) check out Chaparral, just East of L.A. Biggest bike store in the world (true!) far as I know. Loads of stuff ... so no ordering /waiting. They've got most everything in stock, on the shelf.
Mens Street Bike Gear - Chaparral Motorsports
As mentioned above, Temp registration is good immediately, permanent vehicle title (called Pink Slip here in California) takes 2 to 4 weeks typically. But for USA and Canada travel the 60 day Temp registration is legal in ALL 50 states.
With a used bike bought from a private party you keep the same license plate. If you buy a New bike, the dealer gives you a temp plate and temp registration.
With a Used bike private party purchase, you go to local DMV (every town has one) and transfer title into your name, fill in some forms, pay fees and sales tax. Done.
With a dealer sale, you need do NOTHING. They do it all and you receive documents (pink slip, plates) in mail later. This is true weather you buy a NEW or USED bike from a dealer. (a used bike may stay on it's original plates) It's their job to handle registration, pay tax and all fees. (which you pay to the dealer!)
Dealers can also sell you insurance .. but may not be the cheapest ... but it is convenient and is effective immediately ... with NO work (other than paying) on your part.
Beyond this, I don't know much about insurance. I know foreigners have limited choices ... so the dealer option may be good?
The Kiwis and Germans I helped DID NOT buy any insurance here since they had only to make it to the Mexican border (120 miles South of L.A.)
USA insurance is not valid in Mexico, once in Mexico, you buy Mexican insurance. Also, California law allows you 10 days to buy insurance after buying a bike. (new or used)
Some folks mention Progressive
as a company for insurance. But in my experience, they are crooks. So ask around, try to find a fair and inexpensive solution.
YOU WILL NEED to show proof of insurance if you get pulled. Honestly, you could probably show your OZ insurance ... and police would probably accept it. But ... in case of an accident you'd be screwed.
The places and routes you've picked are all really good IMHO. Try to avoid hottest weather. Some of your route is VERY hot mid Summer. Try to time your ride to go into Sept. and October if possible. Best weather. Wise to stay out West. The East and South are another trip. So much to see in the area you've mentioned ... a thousand good places and a thousand fantastic roads.
If you come through Northern California, PM me and I can help lay out a nice route around here. Riding paradise if you know where to go.