Proof that this recursive sequence is Cauchy?

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29.18 Let $f$ be differentialbe on $\mathbb{R}$ with $a = \sup \{|f'(x)|: x \in \mathbb{R} \}<1. $

(a) Select $s_0 \in \mathbb{R}$ and define $s_n = f(s_{n-1}$). Prove $(s_n)$ is a convergent sequence.

Elementary Analysis by Kenneth Ross, 2nd edition p.240

All the proofs of convergence I've seen (not many) start out with "for an $\epsilon > 0$, pick $\dots$". However, to me it felt more natural to put that part at the end, since there was a lot of stuff to develop before I proved that such an $\epsilon$ would work. Is this bad?

I would also appreciate any comments in any of the following areas:

Is it bad to make the proof as conversational as I did?

Is it unorganized or unclear in any way?

Did I appeal too much to the reader's intuition?

Is there a way I could've cut down on the length using this method?

Is there a better method? Thank you.