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I’ve worked with a GA implementation before which I don’t recommend if you want to own your data or if you want unsampled, detailed logs. I’ve also seen a full end to end implementation that uses server log shipping to s3, log parsing and complicated ETL processes which I also don’t recommend due to the sheer effort it would take to build.

I’d say go with something like Matomo (formerly Piwik) https://matomo.org. If you wanted to build your own, I’d suggest keeping it simple. Look at Matomo’s architecture and replicate https://github.com/matomo-org/matomo.