Yup, that model is difficult for Layout to cope with.
It took almost a minute to export it all in high settings with the quality bar in the middle.
It takes less than that with medium or even low settings and it prints fine with both too, so it could be faster.
The thing is that I’m using Layout 2016 that makes a lot of difference as older versions were slower exporting pdf. I had models exporting in 30min or more.
What I always do is that prior to export I cycle all pages forcing them to reload. Without doing that, I’ve noticed sometimes images generated on the back of the vectors are cut in half or somehow incomplete.
As a sidenote but of great importance I would add that yours doesn’t seem to be a streamlined method of using Sketchup for landscape projects.
If you are a Landscape Architect what I’d recommend is for you to work with billboards, 2D cutout trees, Face me components, or whatever you want to call a 2D image of a tree, on a flat component that rotates towards the camera.
You can use them very well with plan views but they might be harder to work on axonometric perspectives like you have.
However, if you do have a Sketchup model with 3D trees and you want to create this kind of visualizations, why not export an image from sketchup and import it into Layout. It won’t have vector info, but it will render very fast, I don’t know how badly you need vector info for an image you proably won’t measure from.
I’d also consider rendering these elevations on a package like Thea render and then overly them with a hidden line style viewport without trees nor shadows and by turning background off in Layout. The final image will look impressive, with soft shadows and photoreal materials, it will render fast in Thea less than 30 secs on my machine, Layout imports images and deals with them swiftly so you can also have it accurate with the overlayed vector vector info.