Have you tried Tinkercad? I think it's a good starting point for sketching in 3d. They have all kinda of geometric primitives and it's fairly easy to scale, add and subtract shapes to build up both parts and art pieces. You can import and export STL files to modify others work and to export to your slicer. It can't really make arbitrary rounded shapes but I haven't found that to be necessary given import/export with other tools like Nomad or a good old filing of the resulting print.