It depends on the type of smartwatch. I have a HW2 and have it paired ok with P30 Pro(Mate 30 Pro before). I also have my google account on the watch. There is a guide for Huawei watches that describes how you can setup/keep the google account on the watch and pair it after with another phone without resseting the watch.
About google pay on the watch...I don't know as I am not using it. Might work since I can open the store on the watch.