Great information. I think you might be over calculating the AH use of the Engel. I have one and have calculated the AH based on 2 amps but it does not run continuously. I have estimated that it actually runs about 1/4 of the time so (2 amps x 24 hr)x .25 = 12AH per day.
I bought 2 lead acid AGM batteries last year but have not had a chance to try them out. I should have about 190 AH of which 90 AH would be useable to keep batteries to required minimum 50% charge.
Firefly batteries cost at least 3-4 times as much as my AGM’s. I couldn’t justify the extra cost.