Protection in China is layered. No single measure is sufficient, and the ones that matter most are the least legal.
Register first
Trademark rights in China generally go to the first party to file, not the first to use. Filing before you start showing your brand around is inexpensive; recovering a mark someone else registered is not.
Write agreements that are enforceable where the factory is
An agreement written in Chinese, under Chinese law, with a Chinese court or arbitration venue named, is worth considerably more than a European template translated. The point is not formality — it is that the supplier believes it could actually be used.
Split what is worth splitting
If one part carries the real know-how, consider having it made elsewhere or keeping the final assembly. This is a design decision more than a legal one, and it is often the most effective protection available.
Be realistic about what is worth protecting. A generic enclosure is not. The mechanism, the firmware or the supplier list might be.