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There’s currently no established way to setup general config key/value pairs which would apply to all stacks, right? kind of like a default config: key

Yes, there is no currently established way to do this using core features. Advice I've seen several times is to have a zero-resource stack with the shared config as outputs. Then use stack references to get that config.

However, since you're talking about config that's above all the stacks, then your preferred language's normal config options are probably equally suitable and have a lower overhead.

And they provide the config in not-outputs.

That is, I recommend not using Pulumi to solve this problem: use your language of choice.