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AmberELEC
Handheld firmware optimized for the Anbernic RG351P/M/V/MP, RG552 and other compatible devices.
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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.
There is also JELOS which I recommend checking out before making any decisions.
ArkOS announcement - https://github.com/christianhaitian/arkos/wiki/Announcement
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.