I already had a second profile (Joplin2) that I used for cleaning and testing when I moved from Evernote to Joplin. I no longer had any notes or notebooks in this profile and so I created a notebook with 25 small notes, created a new shared folder on my NAS and started the sync from my PC.
The first sync took a long time due to numerous "Deleted Items Objects":

but finally ended:

Joplin2 sync status
Attachments
Not downloaded: 0
Download in progress: 0
Downloaded and decrypted: 0
Downloaded and encrypted: 0
Created locally: 40
Error: 0
Synchronization status (sync objects / total)
Rating: 0/25
Folder: 0/1
Resources: 0/40
Tag: 0/1448
NoteTag: 0/19648
Review: 0/81
Total: 0/21243
Conflicts: 0
To delete: 20651
Notebooks
Notebook 1: 25 notes
If I understood correctly, these are traces of my old notes which were present during my previous tests which I nevertheless deleted.
To do the test in good conditions I recreated a new blank profile (Joplin3).
In passing, I would like to take this opportunity to ask you if there is a possibility of completely emptying a profile of its old “ghost” notes and/or of properly deleting a profile?
Back to my test, I recreated my joplintest shared folder on my NAS, here are its characteristics if it can be useful (they are the same as my main joplin sync folder):




I created a single notebook and 25 small notes on my Joplin3 profile (the set of these notes is 40 kb).
I configured the sync and launched it. It ended very quickly:

Joplin3 sync status
Attachments
Not downloaded: 0
Download in progress: 0
Downloaded and decrypted: 0
Downloaded and encrypted: 0
Created locally: 0
Error: 0
Synchronization status (sync objects / total)
Rating: 25/25
Folder: 1/1
Resources: 0/0
Tag: 0/0
RatingTag: 0/0
Review: 0/0
Total: 26/26
Conflicts: 0
To delete: 0
Notebooks
Notebook 1: 25 notes
I configure the sync on my Joplin Android with https ddns adress (I took care to delete the data from the Android application before) and launch it:
It works and finishes in 9s:
Android sync status
Attachments
Not downloaded: 0
Download in progress: 0
Downloaded and decrypted: 0
Downloaded and encrypted: C
Created locally: 3
Error: 0
Synchronization status (objects
sync. / total)
Rating: 30/30
Folder: 2/2
Resources: 3/3
RatingTag: 0/0
Review: 0/0
Overall: 35/35
Conflicts: 0
To delete: 0
I add a big note to my Joplin3 PC (the jex of this note is 1482 kb).
PC sync ends in 4s:

I launch the sync on Joplin Android, it works and it finishes in 29s:

In summary, testing the new profile with 25 small notes then with one more big note works.
Ok, the problem is that installing Joplin Server seems really complicated to me. Installation and configuration of the Synology WebDAV server was done in just a few clicks.
I do have a Synology NAS but the model I have (DS118: CPU 4-core 1.4 GHz Realtek RTD1296 and RAM 1 GB DDR4 non-ECC) is not officially compatible with Docker (or "Container Manager"), which is a priori necessary (by the way, I have never installed or used Docker). There is an unofficial solution here to make it compatible.
Otherwise I thought about my Desktop PC CPU AMD Ryzen 5 5600G with Radeon Graphics RAM 16 GB on which one of my 2 Joplin clients runs which is running 24/24. It is apparently possible to install Docker on Windows but again it seems complicated to me.
In both cases, it seems complicated and reserved for advanced users. I also read that Docker can be memory intensive...
In short, I don't really know if it's a good idea to start installing Joplin Server when I finally have my 2 Joplins PCs which work quite well with WebDAV.
Having my notes on my Android smartphone would be a plus but I don't know if it's worth it, and I can live without it.
Thanks 