Do you need AGM batteries? If you had 70+ amps charging from generator power they certainly have an advantage. With about 10 amps from solar there is not really much advantage besides lower maintenance.
I would tend to save the AGM battery money to get more battery capacity or immediately replace all lamps with LED.
I kinda feel the same way with the battery monitor. Although it will have benefit of telling you if you need to conserve power or ok to live it up. Can't really help tell when to start the generator or how long to run it. The money spent on more battery, solar or LEDs would create actual benefit to keeping the furnace running. Simple voltmeter is close enough for what you are doing and might even be included on the solar controller.
