How can I fix download scale of section background independent of content

Hello Everyone, I have a section with a background is set to cover, and rows and text blocks inside that. I'd love to animate the background image to scale down when I scroll down to that section. Divi animations scale the whole div and its contents together so that doesn't work.

I've written a CSS animation that makes the animation I want by targeting the .et_parallax_bg within the section, but the animation begins when the page loads rather than when the section comes into view.

Scroll down to the third image down the page (blue, showing the sea) and refresh to see the animation which will probably have finished by the time you scroll down to it.

I've seen lots of JS Scroll effects, but I'm not able to make them work, so I wondered if this could be done by using whatever hook Divi uses to animate content when it comes into view.

Thanks in advance