OK thanks @mihi and @4nthony indeed i installed the tile and it does the sync (in a weird way that consists in reminding /e/OS that the sync was enabled (it seems to have forgotten it) … by turning sync off, then back on. 
The feature I was suggesting was a “synchronize NOW in case it’s not done” button. A bit like the “receive” button in email clients.
And the question asked in the other topic still seems relevant: why doesn’t /e/OS do the synchronization automatically, directly?
But in a way, your answer has already helped me make the operation less complex ! (only 4 gestures instead of 6 
… so we can say it’s kind of solved 
. Thanks !