No real battery difference in 6v vs. 12v in AGM. Only difference is series connection is always balanced. Nothing really bad about 2x 12v in parallel either if main power is connected across both. (+ connected to first battery and - connected to the second)
9245 is fine and about max for a 1000va generator. There is a 9140. IOTA has a 30a and a 15a if you want to go smaller.
400w solar seems like a lot of solar for two batteries. Yes you will need it in the winter and I think you may also find more battery will increase your solar harvest to stay in bulk longer.
Definitely swap all lights to LED.