syncthing (or private git repo on trusted server) + pass/gopass/prs (with gpg key on Yubikey or similar hardware CCID/OpenPGP hardware)

It's not for everyone, but it's trivial to conceptualize ALL of it - you decrypt/encrypt plain text files in a dir, and something else syncs them. Your "manager" then becomes a tool with a nice CLI that just does `gpg -e`/`gpg -d` under the covers.

(I'd really like to see a tool that can take a publicly-shareable config file (email, name, country, origin date, keysize, seed) + a secret BIP39 phrase and spit out a GPG secret key, or better, just provision a new Yubikey from the phrase. Then my cold storage could just be my BIP39 phrase, rather than being my BIP39 phrase and an encrypted copy of my GPG key backup.)