Charging too fast, how much is too much?

Hello,

At home I have 600AH worth of 12 volt battery storage, it consists of 6 UB121000 batteries. Here's a link to the battery: https://www.ebay.com/itm/UPG-12V-100Ah-AGM-Sealed-Lead-Acid-Battery-UB121000-Group-27/131399541396?epid=1500545109&hash=item1e9805e294:g:c5kAAOSwwpdW9XRk

I have 2 Epever charge controllers, they are 40A each. I have 4 solar panels connected to each of those charge controllers, they are around 165W each. Link to the charge controller:
https://www.ebay.com/itm/EPEVER-MPPT-Solar-Charge-Controller-12V-24V-Power-Regulator-150V-40A-30A-20A-10A/351875294014?epid=1548119663&hash=item51ed66e33e:m:m4ayhozVYUvR-n1728SejiQ

The batteries are all in parallel (with terminal fuse blocks too). I have the 2 charge controllers connected to the common positive (also fused).

On a sunny day, both charge controllers max out at 40A. They seem to do a curtailment so that no more than 40 amps goes to the batteries. So with both of these combined, I have 80A of charging going on.

I'm considering adding another of these 40A charge controllers and also another 4 solar panels connected to it. I'd attach this charge controller the same way I did the others, through a fused connection to the common positive, and the negative going to one of the negative poles.

My question: How much charging is too much? When is it more damaging than helpful? Also to consider is how very, very little sunlight I get in the winter time too. The low light in the winter time is most of my motivation for adding a third set of panels and charge controller.

If 120A is too much, I can add the extra 4 panels onto the existing charge controllers, they should curtail it back to 40A each as they do now so I'd get the benefits of those only in the winter, but all that electric would be wasted in the summer days.

Opinions? Thoughts? Suggestions?