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https://supabase.com/ is a good alternative, but I don't know if the data is hosted in Europe.
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InfluxDB
InfluxDB – Built for High-Performance Time Series Workloads. InfluxDB 3 OSS is now GA. Transform, enrich, and act on time series data directly in the database. Automate critical tasks and eliminate the need to move data externally. Download now.
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You can look into using appwrite.io which is an open-source (and self-hosted) alternative to Firebase. But you will need to host it somewhere on a cloud service AWS/DigitalOcean/etc
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Or maybe Parse (https://parseplatform.org/), the somewhat forgotten OG of PaaS.
NOTE:
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Hence, a higher number means a more popular project.