I prefer to use external SaaS that allows multiple members of a team to login independently, so we don't have to worry about sharing a single username/password.

I prefer to use external SaaS that allows multiple members of a team to login independently, so we don't have to worry about sharing a single username/password. For things like SSH keys and GitHub repositories, we pay for private organization repos that are shared with the team. API keys, passwords, etc are stored in a private GitHub repository that is only accessible to myself. One interesting aside to your query is "what happens if I get hit by a bus tomorrow?" That is, will the startup cease without its CTO? My solution has been to keep a handwritten password in a safe in my house, and provided an extra key to a trusted third party (my parents). So if worst comes to worst, my legacy will not end when its domain needs to be renewed. Hope this helps, and good luck!