So is it just slower and slower cranking then starting or no starting at all?

I have had batteries that tested good with voltage a cranking amps with tester, but when a load was put on battery, it would fall flat.
I would pull a comparable battery from another vechicle that you know is good and try that, if still no, then go through connections.
And since you are changing batteries, let sit unhooked for 15 minutes for reset, before hooking back up.
Is there corrosion on or around the terminals?
Has there ever been?
The acid could be coordinate g in the wires.