I don't know if you have read the last Process because my computer crashes during its writing :)

Seems you don't make the Boolean and don't erase the inclined lines...before the first Export PDF !
How do you deal with these lines when you colorize?



But ok for the no Boolean if you want the "frame"!

I make all inside MOI only from your file PDF!

About the width of the lines that will be made inside Affinity after the final export PDF from Moi...

But I am agree if the erasing of the no wanted lines was automatic that will be a breeze! :)

Else I have all surface that you want (so no Boolean at the start process;) from the Trimming intersection with the plane! ;)
Even before to have the closed curves! Even before the Explode Move who will permit the close curves process!



You must have something like this


For make the curves close : Join them! (before the Un ExplodeMove)


The Un-MoveExplode...twice 0.5 because ExplodeMove was 4
That is finished! All Closed Curves are in position ready for be exported in PDF (View Top) to Affinity V1


And inside Affinity V1 the round intersections seems perfect ! If not that is not a big deal to connect them in the right place!