You need to go to China National Intellectual Property Administration to go through the "cancellation" procedure first. This program does not need an intermediary, it can be done by itself. Then you can submit your own opinion statement.
Reminder: If a patent agency submits a patent application in the form of "electronic application", the follow-up documents must also be submitted in electronic form. Therefore, if you want to submit your opinion statement, you need to have an electronic application terminal.
If you want to submit it yourself just because of the high cost, you can negotiate with the agency, you write the opinion statement, and then they will hand it in. The cost will be very low, and some agencies have this kind of business.
But if you are not familiar with patent law, you may not be able to answer it.