No, syncing multiple Nextcloud servers in this manner is not possible. Certainly would love to use this functionality myself! Might be one day possible through Federated Circles + mirror federated server
In this case you would want to run Nextcloud itself from a well-powered, resiliant location you can would remotely connect to from whatever device. As you note, it is not meant for your use case as it is designed to be on 24/7.
fwiw, nothing stops you from using multiple Nextclouds. Just keep in mind no network = no nextcloud
This is the way. It has been recommended by Nextcloud staff as well. Many past guides on how to set up syncthing as www-data. Solves issues with lack of connectivity as well as managing massive data sets / full system backups. how to syncthing nextcloud at DuckDuckGo