Download to client based on file size changes only

Hi All.

I volunteer for a charitable organization. We have set up a Nextcloud server with about 600GB of information. Many thousands of mp3 and mp4 files. Next cloud is a wonderful tool, and we hope to benefit greatly from it. We have two questions and would appreciate help:

  1. The affiliated regional chapters of the charity need to keep their copies of the same data current by syncing to the Nextcloud server. Is there a way to configure the client (or the server, as appropriate) so that only files that have a change in file size are downloaded by the remote clients? We have had many cases where only the time stamp has changed (no change to the size or contents of the file; file name remains the same as does the path / folder structure and names). In such cases, very large downloads are unnecessarily triggered. Is there an advanced option somewhere to choose how the decision is made as to whether a file needs to be downloaded or not (timestamp? file size? md5 hash?)?

  2. Many affiliates already have local copies of the data from before we installed Nextcloud. They would like to sync without having to download the whole set initially. This is because they are located in rural areas, often relying on older or slower internet connections, or download limits (100 or 200GB / month). In those cases, is it possible to simply set up their copy from before as the target for the Nextcloud client in the hope that it would recognize the folders structure and files that are already there? In our testing with a much smaller set of data, we noticed that the client was downloading many more files that had changed (but not all for some reason that we could not decipher). We also found that the issue in question 1 affected this.

Many thanks in advance guys.

Bumping this up, hoping for some help. Thanks.