What you can do to exclude some folders from synchronisiation ist the use of a .sync-exclude.lst (with the leading dot) in the root directory of your cloud. Simply add the regular expressions that match the name of the folders you want to exclude.
Unfortunately that feauture is not very good documented and only visible from reading the code but the syntax for such a .sync-exclude.lst is described here:
https://docs.nextcloud.com/desktop/3.13/navigating.html?highlight=sync%20exclude%20lst
So if you want to exclude a directory
/foo/bar/NoneOfMyBusiness
then you should add this pattern to the .sync-excludes.lst:
*foo/bar/NoneOfMyBusiness*
that will prevent the client from synchronizing that special folder.
I hope this will inspire you to find a solution that suits your needs
Much and good luck,
ernolf