I have yet to find a good, complete sync solution that allows me to use different devices (Fitbit steps and weight, Withings steps) and see results on a single location.

https://www.fitnesssyncer.com/ seems ok but very basic charts and information also it is not a complete "sync" but more of a "download". When I tried getting data from Withings and Fitbit and send back to Fitbit it doubled the Fitbit steps - obviously not tracking sources, etc.

Is there any other service that you tried and worked?

Does Strava records steps or exercises only? And can it get data from other sources such as Withings and Fitbit?