Well, I have to be careful not to speak too highly Synology when I haven't used QNAP. I know QNAP has an outstanding reputation too. But, for my needs Synology has been stellar. They have excellent on-disk encryption support and have apps for Android, iOS, etc... there's a very effective Web client that works nicely on the Linux boxes I've tried it from. Plus support for all the file sharing protocols you can imagine. Connectivity won't be a problem.
Regarding offline access to files, Synology allows you to create offline copies of folders using their app called DS File. I've used it on iOS and it works fine. It doesn't have the automatic deletion of old stuff you were asking for, but if you don't mind a little manual management, it'll do the job.
On backups, you've got a few options. With an existing WD NAS device, you can just schedule a backup job on the Synology and it will back up to the WD device. Also, it supports some nice cloud providers so you can (selectively) backup folders to the cloud, including built in support for encryption. I don't remember the name, but I know they have out of the box for a EU or UK based cloud provider. Might be all you need.
Perhaps someone more familiar with QNAP can comment on what their support is like. I expect it's probably quite similar to Synologys.