If I want to go to Holmesglen Institute to study chef vocational training (for future immigration), is it okay to have a technical secondary school degree?

It seems not possible for technical secondary schools. If you graduate in Australia, you are required to graduate via other means.

The school you mentioned is pretty good, equivalent to a junior college. Now it seems that the duration of schooling is two years, but you have to look at how much savings your family has, 500,000 is enough.

I think your first step is to send your Chinese scores to Holmesglen and apply for a degree. If it is given to you, it is not too late for you to make other preparations.