How to keep synced photos only in synced collection?

I'm not really following your workflow here, especially where you are removing images from the collection. That is important to know, as there will be different options presented depending on which app you are using to do that removal:

If using LrClassic, when you try to remove an image from a collection you will normally be presented with a dialog box (which may not appear if you've previously selected the "Don't Show Again" option), which asks if you also want to remove the image from the All Synced Photographs (ASP) special collection as well. However, that dialog will also tell you that the images will NOT be removed from the ASP collection if it exists in more than the one collection that you're trying to remove it from.

Now, understand that simply removing the image from the collection in LrC will only cause the image to be removed from the equivalent album in Lightroom or LrMobile, it will still exist in All Photos in those other apps, i.e. it will continue to exist in the cloud.

However, if you remove the image from the ASP special collection that will result in the image being "unsynced" in LrC (it will remain in the LrC catalog however), which in turn will cause it to be deleted from the cloud.

If you remove the image from an album using Lightroom on the desktop, it will only remove it from the album and will not delete it from the cloud (so it stays in All Photos). There is a separate action if you want to delete an image from the cloud using Lightroom on the desktop. Using LrMobile it's basically the same as Lightroom, except that the "remove" option presents a further option to either remove from the album or delete from the cloud.

Another thing to understand is that if an image is in the cloud it should also appear in All Photos in all connected Lightroom clients (as well as the ASP in a synced LrC catalog). It's not possible to remove an image from All Photos in one Lightroom app, while still having it in All Photos in another Lightroom app.....it's all or nothing in that respect.

Bottom line: if you want to remove an image from the cloud, you must either delete it from any one of the Lightroom apps, OR you can simply remove it from the ASP special collection in the synced LrC catalog.