A good battery monitor is all you need. There is no reason to swap the solar controller, unless you determine it is not charging the batteries properly.
At this point you need to fully charge the battery, and them monitor the system (current, voltage, etc). You shouldn't trust the dumb lights on the chargers, nor should you trust voltage only SOC measurements. Your best friend is a volt meter, learn what voltages are appropriate for charging and resting, watch the current for a few charge cycles. With a properly setup SOC meter (with shunt or equivalent) you can actually determine if your are getting a good charge.
Even without an SOC meter, you can simply track the voltage during charging, and occasionally verify its getting to the correct level for the prescribed amount of time. It easy for things like bad connections, shade, etc to dramatically reduce charging.
As far as charger size goes. You can charge a 500AH battery with a 1A charger, it just takes a very long time. Some smart chargers will error out after so many hours if the battery is very large. In that case you just reconnect the charger and resume the charge.
I will reiterate again. DO NOT TRUST RANDOM LIGHTS. You need to physically verify the voltage the battery is seeing. Current measurements are also helpful. Many chargers are simply not set up properly, and its incumbent on you to know what the battery needs, and confirm it is getting it. Simply looking at the lights flashing will provide little or no useful info, unless you know what votlages/stages etc those correspond to.
Personally I prefer automatic chargers with three stages. 1st stage is constant current until the absorb voltage is hit. Second stage hold absorb for a set period of time (hours). Third stage is float.
