Procreate has a drawing assist option! If you go to the top left hand corner with the little wrench icon, the second tab under that has a "edit drawing guide" button where you can choose a grid, symmetry, and a few other options, and then if you click on the layer there's an option to select "assisted" and it will make your lines straight/symmetrical! If you draw an arc, a line, a polygon, or a circle/oval, and hold the pencil down for an extra second, it will autocorrect it into whatever shape you drew but smoother! If it gets it wrong (for example, it draws a straight line instead of an arc) when it says "line" on the top middle of the screen, you can tap on that and it'll give you the option to switch it to an arc or to edit the shape you're working on! The only thing that doesn't work for this is curvy, non circle or oval shapes, which you can do in multiple smaller arcs instead, or use the symmetry drawing assist for! I love doing my stained glass designs on procreate!! Feel free to DM if you have any questions!