Blender Mesh Modification for 3D Printing

I need Blender meshes (STLs) modified for 3D printing. The final models will be printed using plastic filament (PETG).

Requirements:
- Fix holes, thin areas, and other printing problems
- Resize or adjust the shape for specific uses
- Split the model into separate printable parts
- Hollow out sections as needed
- Make a mold too

Ideal skills and experience:
- Proficiency in Blender
- Experience with preparing models for 3D printing
- Attention to detail and problem-solving skills