I literally spent months evaluating this exact question and wish Id found this guide on choosing a crypto custodian earlier https://dessertscapital.com/factors-to-consider-when-choosing-a-crypto-custodian/ . Beyond obvious security certifications, the critical factors are: insurance coverage details (many exclude certain attack types), withdrawal policy transparency (how quickly you can access assets), and whether they use multi-party computation vs traditional cold storage. The articles "red flags" section saved us from two providers with poor disaster recovery plans.