Instead of splitting the key into substrings or using Reed-Solomon codes I’d recommend something like Adi Shamir’s secret sharing scheme. Using something like

Instead of splitting the key into substrings or using Reed-Solomon codes I’d recommend something like Adi Shamir’s secret sharing scheme. Using something like that allows you to say “split into seven shards but only require three to reconstitute”, and possessing less than the minimum number of shards yields no new information.