I got it working, i think i was just over complicating it after hours of going around in a circle.
If others have the same issue, what i did to get this working was to use Matomo tag manager to create an event in my case on form submission, and then make a goal where the goal triggers on the event category that matches the one i set up in MTM
Super simple now that ive done it but took me most of yesterday to figure out!
Thanks for the help Stefan, when you said make sure IDs are linked got me thinking hah.