Converting the bodies to 2D as you originally said would be a good idea but I can't explain to you how to do that very well. There are lots of videos on youtube explaining how to do it.
One tool you can use to create 2D surfaces from 3D geometry is the midsurface tool.
The same goes for defeaturing, plenty of tutorials on youtube.
If you want to post an archive of your FEA I can take a look at why your simulation might not be solving.
Regards, Josh
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