maybe this from the above! :)
Explode Move = free surfaces in the space
Select Edges and move them = I have only closed Curves !
UnMove
UnExplodeMove
Export PDF

here just export PDF for see if the closed curves works! :) (before the UnExplodeMove)
Filling in Aff 1 works now like a charme!


I believe i am near an easy process! :)

Tips
Better if the original objects are Boolean Union and generator vertical line of Cylinder hidden

The complete easy final Process! :)
Of course each closed curve can be Style colorized and Named inside Moi!
for have directly named Layers inside Affinity V1! (no made here)

With this http://moi3d.com/forum/index.php?webtag=MOI&msg=3334.2
you can name automatically some Styles (so futur Layers) as you want
for have something more elaborated than "Default" following! :)
( Calque = Layer in French)

Intersection with a plane of the Imported PDF






else i have refound a Close Multiple Curves Command :)
https://moi3d.com/forum/index.php?webtag=MOI&msg=2752.1