Using Scribus, Create a document with the dimensions of 1920 by 10,000. Make the page black, Your page size will differ, but 10,000 is a good starting point. My end credits are 3 minutes long and are 1920 by 14,000.
Add your credits, apply formatting as needed. I suggest using the text editor for tabs and for making other complex edits.
Add images as needed.
Export as a PNG.
Add to magnetic timeline in FCP.
Crop, use Ken Burns.
Move the start to the top above the credits. Move the end to the bottom. Right click and select linear.
So obvious that I didn't think of it !! Will try that (actually with InDesign cause I have it) cause I really don't like Manifesto (since it lets you fly blind with the layout while you're in its editing interface) which I use currently.