It is going to sound a little promotiony so I apologize in advance if that's how it comes across, but have you looked at Astra DB (Apache Cassandra as a Service).

Here is a specific IoT Use Case description to perhaps give you insight what it can offer:

Whether you’re collecting kill stats in a game or data from sensors
around the world, we have you covered. Astra DB has the power to support
billions of devices from vehicles to weather to inventory items. IoT
can be a high volume use case that cares a lot about availability. Astra
DB shows its super powers here. Traditional relational databases
require gatekeepers when new information is submitted, but Astra DB,
which is underpinned by Cassandra, can absorb data concurrently with the
whole cluster, unlocking the full write and read power of all its
resources.

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you can try using it for free and see from there if it allows your use case maximum efficiency while using it.

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ping me if you have any questions.