You did a factory reset already, so software-wise I would say you could try
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Installing from scratch to rule out the current state of the OS as a cause (a factory reset doesn’t touch the OS, it just deletes user data and Apps).
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LineageOS, because it’s still Android and you can set up and use your stuff in the same way as before, but at the same time it’s different from Fairphone OS and might behave differently concerning those reboots.