A bit surprised you need CURA 4.6.1
I print everything and its all in PLA using later CURA 5.7.0 with no problem.
The picture you posted are quite low quality and do not really show the problem you describe so it is hard to recommend a possible solutions.
Printed structures in PLA are relatively brittle
across the layers but about 5 times tougher
along the line of the layers.
This tissue covered electric plane has a conventional "stick and tissue" wing construction except
all the wing & tail parts including the spar

are printed in PLA and glued together!
The fuselage is also printed except the tail boom.
Its all about ensuring the PLA is laid down for each part in a direction best suited for the loads it has to carry.
It flies pretty well. A bit heavier than if it was done in balsa but then PLA is at least twice as heavy to start with.