Nextcloud has more than just file backup to it. It has a whole host of other apps associated with it. This details what I use to get a Mac app for Nextcloud, it may just be a web wrapper but you can access all of your Nextcloud apps.
Begin by downloading the FluidApp for mac from Here. Then once you create a FluidApp, for the URL: enter your IP address or domain name of your Nextcloud installation, name it Nextcloud and have it use the website favicon so it will pull in the icon. That's it, that's all it takes and now you have a moveable, pinable, app that launches your Nextcloud installation.