The alternative would require the user to enter a decryption password on every system start, like some wallets do, which is a bit of a hassle.

The downside is that you need to type a password - the upside is that you don't need to type any extra password, since you are already unlocking whatever wallet you are using anyway (unless you don't use one - which is a whole different problem on its own).

If at least there was “one obvious way of doing this” across platforms,

For wallets I found https://github.com/hrantzsch/keychain/, but TBH I don't think OS password managers would be the way to go here (at least not if you want to support CI systems and building in containers). Something based on age would be far more flexible, and could leverage existing ssh keys (which I'm sure some people store with no password protection - which, again, is a whole different problem on its own).