Learn English – Adjectival step by step

At bottom your problem isn't too many prepositional phrases - it's too many nouns.

Nominalization has been the bane of "scientific" style since the heyday of positivism: nouns, 'things', are somehow regarded as having more 'reality' than verbs or adjectives, even when the 'things' are just syntactical transformations of the deprecated 'actions' and 'qualities'.

Turn some of those gratuitous nominalizations into verbs or adverbs or adjectives and the problem goes away:

What metrics are appropriate for evaluating how accurately a difficulty level has been predicted?

Liberate yourself from hegemonic nominals.

We use "a first step" to convey the idea that there may be other options for "first step", and that what is being offered is just one of them. "the first step" would be used if a set of steps has already been identified.