Michiel van der Velden,
If you check again with the ModelingSettings> Mesh Check, you will see that there were two bad polygons. I used the Mesh> Remove> Optimize Tool (cogwheel) with a tolerance of 0.01cm, and they were gone. See the link above about Bad Polygons.
In the UV Editor view, I switched on Overlapping UVs. Editor View Menu (while in UV Edit Interface)> View> Overlapping Polygons.
Overlapping is, by itself, not a problem, sometimes even wanted. What it means is simply that the several “picture-frames” are on top of each other. This will result in similar or even equal picture content in the 3D polygons. Think of a label that should show on both sides of the car, then you can have that only one time in the image. But if the surrounding area of that label should be different, it might be better to have that label twice in the texture-image.
I found some NGons and some extra polygons in the bottom area, I assume they are not needed.
I would suggest two options. One is more in alignment with the organic shape. It splits the main body at the top and bottom. Two UV meshes, and if you use any Carbon Texture, this might look best.
The other one is more box projection oriented and keeps the distortion to a minimum.
You can use both of course to a certain degree, for paint and for the labels the car might have.
The right-sided UV of the Material tag will take over.
https://help.maxon.net/us/index.html#TUVW
Let me know which one you will use if there is a question about labels at all.
The project file, the main body
https://www.amazon.com/clouddrive/share/py7eq8Lpt1gHFyF70IUwOAeR3CYjvgdrTdS2wvbwSzA
I would resolve the Boole and the other parts, to get a pure polygon model, then start texturing.
My best wishes.