I installed AGM telcom/server batteries in the previous MH a DP,
Capable of 500w per cell AKA 3000 watts per battery until discharged, about 10 minutes, but would be totally discharged, but this is Tested what it could stand spec for when new, Not recommend design use
And was very happy even after several years of use
But I did not do back to back electric kettle water boiling
Or run the A/C
I think your use pattern, has created a physical/ molecular barrier in the plates , that cañnot carry/pass the amount of current you want to use
Just like any other lead battery, you need enough Total capacity, that max power use does not exceed designed power use per cell
If you want long life trouble free use
If not enough storage space for that
Then you have to expect, replacement when performance drops below need
