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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
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dovecot-core
Dovecot mail server, modified to work with fragtion/syncthing. Customized source code can be found in 'fragtion-master' branch.
I don't use it but https://syncthing.net/ is often mention for scenarios such as this. I think it would be the simpler solution.
Dietpi (based on debian/RPi OS) comes with a handy software installer, which also supports nextcloud and many more apps. https://dietpi.com/
Regarding Syncthing I've seen a couple of abuses of it in datacenters. A friend used it build a poor man's CDN with 80TB of data in sync across multiple geographies. Others have used it to have redundant mail servers (https://github.com/fragtion/dovecot-core) and even to build redundant Nextcloud clusters (https://bayton.org/2017/06/experimenting-with-clustering-and-data-replication-in-nextcloud-with-mariadb-galera-and-syncthing/).