A quick and dirty, literally, way to check ABS is to get one side of the car on dirt or gravel, the other on solid pavement and mash the pedal. If there's no pulsation, abs isn't working. Here in MN, we do that all the time in winter on icy roads, although it's usually not planned. We have an '03 SE wagon with ABS (I think it may have been an option, traction control was, are you sure your car has ABS?) and it pulls down from 60 mph on dry pavement with brakes to the floor no problem, no pedal sag, obvious pedal pulsations all the way to a stop. I've done that repeatedly and it's exactly the same every time, the brake system is really good. The pedal isn't super hard, but it's not supposed to be. It should not be spongy though, sink during steady hard braking or be different from stop to stop.
If it definitely does have ABS, pad differences rotor runout, etc won't really have any unbalancing effect but likely will reduce overall braking power; the system works by comparing wheel speed to maintain even braking. I had a rear pad debond once and the system still worked but made a wild grinding sound. One new rotor later...
We had a '94 SE sedan with discs front, drums rear that did something similar to what you're describing. It didn't have ABS but did have a level sensing proportioning valve in the rear activated mechanically by a rod that biased the back brakes more in proportion to increasing rear load. Except on very slick surfaces, it was impossible, I mean you couldn't, lock the front brakes, but the back ones would lock under very hard braking or on slippery surfaces. It was a scary car under hard braking, seemed like everything happened in slow motion with your foot through the firewall!
Anyway, at 140,000 miles, the brake lines rusted out and I replaced them. Since all the fluid ran out during the repair, the brakes were bled and I had to do it 2-3 times to get a hard pedal. The weird brake bias mostly disappeared and the car stopped much better afterward until we sold it a year later. I know I bled the brakes when the front pads and rear shoes were replaced at 80K, but just enough for the black stuff in the fluid to go clear and that didn't change the poor braking or pedal feel, only repeated bleeding did the second time.
I tend to agree that there's still air in the system. At least eliminate that possibility before going deeper because it can mask lots of other things. If you decide to bleed again, be sure at least the amount of fluid estimated to be in each line is purged before moving on and the bleed fitting is really tight before you say "Up"; your helper takes their foot completely off on "Up" and holds the pedal tight to the floor with no movement on "Down." Wait until you hear them push to open the bleeder. Have a quart of new fluid, it's cheap and allows for mistakes.
Ask your helper what they're doing once in a while, all this may sound silly, but to most folks bleeding brakes is like watching paint dry; it's easy to get mesmerized hearing "Up/Down" over and over and accidentally lift the pedal a little when the bleeder's open.