Ok so the IMF October edition puts the estimate for 2020 at 102,491.039 billion (102.49 trillion) yuan
At 6.90 that equals $14.853 trillion so you are right a little under $15 trillion for mainland China.
Some people estimate <6.40 average dollar-yuan rate for 2021. If we assume 6.40 to be true and it doesn't seem all that unlikely, and we tack on 2% inflation and 8% GDP growth like you did then China at $17,562.54 billion ($17.56 trillion) will add almost $2.7 trillion to its GDP this year.
I don't even think these are optimistic. They should be seen as a median estimate!
China is likely to add more to its GDP than the entire economic existence of UK or France. Won't even count India since there seems to be pretty widespread accusations from international orgs of fraud. Would not be surprised if their GDP is closer to Italy than to UK.