Aaaaand I have it working ! I used a code found in another post:
let activeDecliIFrame = document.querySelector('#activeDecli');
var timeline = gsap.timeline({ id: "masterTimeline" });
var tl1 = gsap.timeline({ id: "timeline1" });
var contentWindow = activeDecliIFrame.contentWindow;
var tl = contentWindow.childTL;
tl.pause(0);
tl1.to(tl, { totalProgress: 1, duration: tl.totalDuration() })
timeline.add(tl1);
GSDevTools.create({ container: '#GSDevToolsContainer', animation: timeline, globalSync: false });
I don't have a clue (yet) on why this code works and mine not, but god I am happy (I spent more than six hours on this one !). Thank you for your help anyway, it helped me understand a little better what's going on
Mamorukun's post in Unable to make GSDevTools work on an iframe was marked as the answer