Since you compose geometric pictures, then surely you'll need a vector image
editor like Inkscape but not a raster image editor like Gimp or Krita, etc..
You can try to find some good vector image editors by searching something
like "list of vector image editors".
Please note that vector image (editor) and raster image (editor) are very
different. One graphics editor may be good at composing one image type but
not both.
But if so, tablets and digitizer pens won't help you much. They are designed
primarily for digital painting (raster images). You may find a
high-resolution screen and a good mouse much more useful.
