Having done only the trip to Panama and back, but multiple times I can only address the segment up to that end. From reading here I do gather that South America is similar, but leave it for others to address. You are correct, no Carnet needed

For the USA you do need insurance for public liability . For Mexico too you need it and also for the CA countries. Some will sell it at the border as part of the process to gain admission and it is not very expensive.
For some like Guatemala it is good to have but not possible to buy at the border or at insurance agents (!!!) so best have it arranged on internet beforehand.
For Mexico and USA you will need to buy it from private insurance sellers . Shop around for best deal.
For mexico and CA countries you need to buy a tourist permit, not expensive.
For Mexico you need to buy a temporary vehicle import permit as well as a personal tourist permit, total about $60US , and you must post a REFUNDABLE TVIP SECURITY DEPOSIT, max.$400US which you get back when cancelling the TVIP and removing bike from Mexico.
Belize has a special small tourist tax levied at the time you exit the country.

Border crossings for ENTERING a country all follow the same steps of Immigration, Vehicle Import , Fumigation (not always) and Aduana where they double check all documents.
Leaving a country usually takes less time and has no fees to pay ( except Belize) You must go through the routine of Leaving one country then march directly to the other country to process as Entering
There are small fees but nothing spectacularly expensive. Consider it as your tourist tax
Mexican borders are straightforward and well organized and fairly rapid . THe CA countries each have their peculiarities and can take on or more hours .. The PanAm crossing at at El Amatillo ,Honduras is the worst and notorious for long looong delays.

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93