@ Peer ... SVG : it's a little nightmare! :)

I have maybe a process (in PDF) but we must hide these yellow lines by hand
so not sure that is a valid one that you have expected!





The Process some similar than previous !
From an Moi Object
- Boolean the solids
- Hide the lines no wanted
- Export as PDF View 3D for have any position wanted!
- New - Reload the PDF - View top (so we have the position of the original object in a plane ! )
- View Top - Intersect with a Plane
- Keep only the Object kill the rest
- the tricky Part! ExplodeMove (4)
- Select Curves (use the Browser) Kill them
- Select & Copy Move the Edges (use the Browser) become Curves! ) - Join these Curves = All curves are Closed!
- kill surfaces - put the closed curves at the original position
-UnExplode Move
- Export as PDF the closed curves View Top
- Open inside Affinity V1: colorize as you want all the Closed Curves!
et voilà! :)
to continue...but I leave the SVG for the moment too devious! :)

Else something like that :)