There are a few things that stand out based on OPs description.
- low mileage
- gentle driving
- brake pedal requires pumping
- pulls to the left under braking
- vibration under braking
Even with cheap pads, this type of mileage and this type of driving wouldn't result in high wear. Take a look at the pads to see how much they have actually worn.
Applying the brake pedal with no slowing of the car, having to pump the brake, is not normal or a fluke. Likely air in the lines. Maybe a failing master cylinder. I would do a full brake system flush. You can look this up on YouTube if you've never done it.
When swapping brake pads and pushing the piston back into the caliper, you should open the bleeder valve and catch the fluid, versus pushing it back into the master cylinder.
As has been stated, you should assess the guide pins on all four corners. Remove, clean, fresh grease.
Assess the rotors for pad residue. You may see places on the rotor surface where the pad left pad shaped residue. If this is present, that is likely what's causing your vibration. You can pull the rotors and clean them. Hit them with brake cleaner and steel wool.
Once everything is checked, flushed, and mounted back up, bed the brakes.