Definitely switch to 12" tires -- they are readily accessible, as you found out, at KMart, Walmart, Pep Boys, etc. and easier on the bearing. Buddy bearings are very important as well.

On insurance, I once used Boat US and another I can't remember, and then I ran into All State Ins and they are really inexpensive -- Like $150 a year for damage and liability.
Be sure to tell them it is a beach cat.., like a small Hobie. Otherwise they don't know what you are talking about.
Good luck with the new toy,
Rick