I made one for our company using AWS Kinesis Firehose which I thought was really good having used GA, Mixpanel and Segment before. Shame we haven't been able to put it into more wider use. Extremely simple and very robust, to deploy it you just have to run the CloudFormation stacks with Sceptre in a single command and then add the client library with some event listeners for clicks, pageviews et cetera. I'd love to be able to open-source it but I don't know, should think through the benefits and disadvantages of both with my CEO. Probably couldn't get customers to pay for an expensive custom analytics platform if it was open-source.

Having spent some time on this I'll just say that don't overthink it. Over-engineering such system is way too easy while the actual benefits might not be that great. Sure if you're receiving a lot of data there might be some pitfalls to be aware of eg using proper bucket partitioning with Athena for queries.