It's an excellent idea to print the bolts horizontally with some flat area for avoiding support material. Small overhanging thread tips (teeth) are still a problem for 3d-printing even if it's only 30°. But if the bolts are split in two halves, there are no overhangs at all! The standard M12 threads have a 1.75mm pitch - that's at the limit for a 0.4mm nozzle. I recommend to design some screw with 2mm pitch (e.g. M16).