Only use Yunohost internal (app) update system for that, because integrating a software in Yunohost takes some work (and for Nextcloud, quite a lot), and a manual update would probably break your Nextcloud. Also, after a new release, especially the major ones (Nextcloud 19, 20, 21…), it takes some times for the packagers to upgrade Yunohost package.
However there is no way to be notified if an upgrade is available, except being notified of all pull request on the github repository, or subscribing to the topic here [Nextcloud] A cloud for personal or group use (but maybe announcements are not always made for each update).
So you have to check regularly (using Yunohost internal update tool, or by checking github/the forum).
Nextcloud notifier might be useful for you to be informed when a new official version is available, then you can expect the Yunohost package to be updated in the following weeks (most of the time).