Nextcloud is protected by whatever form of encryption you configure your web server to use, which can include "quantum safe" forms. And further, you've already applied encryption to your drive, so it is encrypted in storage. While your hardware probably doesn't have any "quantum safe" encryption schemes, you could certainly use a software encryption to achieve that.
Further, when it comes to "quantum safe encryption", you are dealing with future theoretical things, which is probably overkill. Ask yourself this; what are the chances that someone is going to take a dump of all of your data-in-flight in order to maybe one day decrypt it? Same with steal and store your hard disks.
Personally, I'd take the "I control my data" as WAY more important for security than anything else you've suggested.