If the assembly is a standard machine, it will not be too tired after proficiency. If it is a non-standard machine, you need to accumulate assembly and debugging experience.
The salary of machine maintenance is not as high as that of assembly. I have been assembling for half a year now. There are about 3,500 overtime classes outside Shenzhen customs, and our team leader is more than 6,000.
But if you want to be a fitter, you should assemble it. Can it be repaired after assembly? And there are electrical, hydraulic and pneumatic things in the assembly, which is enough for you to learn.