It's normal to not be good at a new lift, even if that lift is a variation of a basic exercise that you already do. It takes time to develop technique, strength, self-knowledge, and self-confidence in a new kind of squat. Each kind has its own particular needs, each of which exposes a different weakness in the athlete. So it's not surprising that 60-100kg gave you trouble even though you can hit 180kg in a variation you're well-practiced in.
It's good to focus on a particular kind of a lift for a long while, because it's the best way to develop general strength and other attributes. But that means accepting the fact that you won't be able to push similar numbers in other variations of that lift.
That said, front-loaded squats (front, zombie, Zercher, goblet, sandbag...) and back-loaded squats (lowbar, highbar) are quite different, and develop different qualities. Of those, most people consider the lowbar squat the best way to move the most weight. For instance, it's common to expect one's front squat to be about 80% of one's back squat if the two are equally trained. The Zercher is basically a more difficult, painful, and awkward front squat, and you haven't trained it ever before, so it shouldn't be surprising to discover your max is about half of your well-trained back squat. Even if you trained the Zercher squat rigorously for some time, it's unlikely you'd ever break 120-135kg in that lift without needing to push your back squat much higher.