You're right that, when your own server is trustworthy, fully self-hosting removes the need for SRI and integrity manifests. But in the case that your server is compromised, you lose all guarantees.

Transparency adds a mechanism to detect when your server has been compromised. Basically you just run a monitor on your own device occasionally (or use a third party service if you like), and you get an email notif whenever the site's manifest changes.

I agree it's far more work than just not doing transparency. But the guarantees are real and not something you get from any existing technology afaict.