Wow, that is quite a crazy touring bicycle. Props. Add me on WeChat and I’m happy to connect you with some other cyclists in Chengdu, if you are interested.
About the panniers: I think you can buy them at the Giant store on Kehua Bei Lu, but you can definitely get them on Taobao and JD. The ones which I’ve used and recommend are LKLM, they are about 600 RMB for the pair.
For components and service, I recommend the Giant shop mentioned above. Since the Natooke bicycle shop has closed (sad event), that has been my go-to shop for service and it is good. For components I usually buy online since there is much greater selection there.