Which prospect is better, product structure design or traditional machinery industry?

The former-better prospects. Actually, you are talking about two directions. If you do well, your salary will be high. Compared with designing electronic products, the entry requirements are lower, but you need a wide range of knowledge and have higher requirements for making machinery (you have less opportunities to design products yourself). After working as a mechanic for several years, it may be difficult to switch to product structure design.