End to end could still - especially with a company like Google - include data collection on the device. They could even "end to end" encrypt sending it to Google in the side channel. If you want to be generous, they would perform the aggregation in-device and don't track the content verbatim, but the point stands: e2e is no guarantee of privacy. You have to also trust that the app itself isn't recording metrics, and I absolutely do not trust Google to not do this.

They make so of their big money from profiling and ads. No way they're not going to collect analytics. Heck, if you use the stock keyboard, that's collecting analytics about the texts you're typing into Signal, much less Google's RCS.