Starters will often go out over time, mimicking battery symptoms of slow or weak starts, with periods where they will not start at all and then working later. Take care when cleaning connections or replacing the actual start wires because sometimes this will have a "bandaid effect" and the vehicle will start, probably stronger then it has in a long time, and a week later it's dead and never working again until the starter is replaced.
That lesson learned on Jeeps lol. Jeep starters are notorious for the sluggish behavior I described, and many times a new starter cable delivers so much "umph" to the dying starter that it quits a few days later.
I would bet if you throw another battery in the behavior will continue, and pretty soon you will have a dead starter. Hopefully at home.