It's Not Rocket Science
This app only exists because of Fitbit being shortsighted. Enter "SyncSolver" to sync your Fitbit data to Apple Health. The process is relatively simple & this app seems to get it right "sort of". On first run it synced all my readings from Fitbit and for days where there were no entries it just went ahead & guessed the reading to fill in the blank. This is really annoying as I now have several thousand false readings for these days. 2 years later and this bug has not.been fixed. The way to work around this is to delete all "sync solver" data from the Health App and then re-sync. Apart from this issue and the fact that it can only do one reading a day it seems to work pretty well.