Can you disble sync once set up? There does not appear to be a “none” option.
Also the method you mention could mean that when you re-connect later Joplin will see the empty sync target in your cloud storage and try to delete the notes within Joplin if it is not treated as a “first-time” sync…
A way of clearing out Joplin both locally and on the sync target and then re-importing a copy of the notes is decribed in this post. You can just skip the re-linking to sync stages until you want to reconnect again.