In general, you need to work at a larger scale for something like this. Model as if millimeters were meters. If your slicer software will let you define the units, you can leave the model in meters when you export the .stl file. Otherwise, either Scale down or use the Dave method. (Search the forum for the tutorial Box made on that.
There are a number of different ways to skin the surfaces, One would be to use an extension from Sketchucation called Curviloft.
Also pay attention to the numbers of sides in circles and arcs. Some of them are excessive and some use odd numbers which makes it difficult to work with the model. At this stage I think it would be easier to start over with a clean model. Since the thing is symmetrical around its vertical axis. I would use the origin and the blue axis line as the center of the model.
FWIW, I redrew your model from scratch. I used Curviloft to skin over the surfaces on the side. Since the model has symmetry, there’s no need to do more than one side and one of the cut off corners. The extension creates groups of the geometry which can then be copied around the center using Rotate/Copy to make the rest of them.
Then the groups are exploded and the central hole formed by drawing a circle and using Push/Pull to push it to the bottom. This should result in a solid component.
I use i.materialise to check .stl files. This is what I get from the model I created.