At first glance, this looks like it could be missing layers. The following is from All3DP.com's article "3D Printing Troubleshooting All Common Problems"
Some Layers are Missing
What’s the 3D Printing Problem?
There are gaps in the model because some layers have been skipped (in part or completely).
What’s Causing this 3D Printing Problem?
Have a look at Chisel infills there are very strength optimised, limitation is currently that they must be single line width, but multiple-lines thick infill will be supported soon.
After that, it's very simple, basically setting double corrugated infill with corrugation size 1 for your rectangle model.
Yes you can!
To have stronger prints you would have to choose the correct direction of filament deposition paths/traces. This answer demonstrates changing the direction of the filament path in Ultimaker Cura slicer.
To do this, it requires some tinkering of your model and choosing the correct slicer parameters (decimals aren't allowed in changing the ...