FTP is not updated from external applications, at this time. I don’t know if it’s due to Garmin not allowing it. However, you can match the data, as per my explanation, as a once off update to ensure that your (time in zone) TIZ data is correct.
Weight can be updated from Garmin.
You can click on the DOWNLOAD OLD DATA (providing it’s ticked in the wellness data options - pencil icon), but this is limited to 12 months, which Garmin only allows.