Now that you mention it, we hadn't decided anything about the most crucial part.
Literal
That-if-say, core-essential [gen-の] part [obj-を] anything wasn't-deciding.
そう言えば is the stock discourse marker 'now that you mention it / speaking of which,' used when a previous comment triggers a memory. 肝心要 (かんじんかなめ) 'the crucial / core essential (point)' is a set compound reduplicating two words that both mean 'crucial / pivotal' (肝心 and 要 each meaning 'the key point'), yielding an intensified 'the very crux.' 何も決めていなかった '(had) decided nothing at all.'