Bossing is not a ladder
Every boss is practice for a handful of mechanics. Find where you are, then pick what you want to learn next.
Why not just use a ladder?
Ladders are useful: they simplify progression and tell you where to go next. But they answer only one question, which boss should I kill next? The question that makes you better is which skill should I learn next?
Movement, prayer, gear switching, resources, extra enemies and timing are all skills. Scurrius and Amoxliatl teach the same two, one random and reactive, the other predictable and rhythmic. Neither is the wrong boss. You're choosing which version of the skill to practise.
The ladder is just how the web looks while you climb through it: one route drawn on a map with many paths. Pick a goal, then pick the bosses that train what that goal needs.