I'm also familiar (but never used) Envkey, which I think might also be from the YC alumni? but I'm not sure...
Shameless plug: I created an open-source tool called envwarden[0], which is really just a simple wrapper around the Bitwarden[1] CLI (also open-source). envwarden helps you manage your server secrets and other variables inside your Bitwarden password manager.
Definitely not as polished as neither Doppler nor Envkey, but just another (open) alternative I guess :)