I have to restart the watch every time I finish any activity to have my data synchronized

Have you tried deleteing all activities? Sounds like a classic case of insufficient storage space.

Hi! I did this the day I wrote this text in an attempt to normalize the problem but it didn't solve it either.

Have you connected the watch to a computer, gone into the Acivities folder and checked if the activity files have actually been erased? If not, you could delete all those FIT files in the activity folder and see how it works. 

I'm assuming you have done a soft reset and not a hard reset, though you mention hard reset. A hard reset brings that watch back to factory settings and does a complete erase. You may need to do that in the worst case.

Contacting customer suppport might help.

I also wonder if the unsuccessful software updates are interfering with your acivity updates. Have you tried disabing automatic software updates? If you've got a stable version running, no reason to upset that apple cart. If you wish to push through with the software update, have you tried using Garmin Express instead of Garmin Connect?

Worst case, looks like you are in for a Hard reset. That's an essential survival skill in the Garmin ecosystem today, with all their unstable updates. So better to master it.