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plex-db-sync
Synchronizes the database watched status between two Plex servers. This includes watched times, and works for all users on the system without the need for tokens.
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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.
As you already said, the Plex supported way of syncing to trakt is only one way so from plex to trakt but there is also a plugin for that, the plex trakt scrobbler. Unfortunately, plugins are not supported anymore and while you can still use some of them, they might be removed any update. This means, you could use that plugin to sync something right now but this might not hold for long.
From what I can find there is a Plex-db-sync here and there is a python package plex-watched-sync.