My experience with QuickSight has been pretty negative. The overall UI/UX is pretty meh. If you're embedding it in your product you may be better off generating your own reports, in app.
We're talking about a few-hundred megabytes of data for all of the customers that these reports pull, but that's also for the past 15 years. We do have like 25k customers, which shrinks how much a customer can pull in even further. One last point is that we already de-normalize the report data into its own table specifically for these reports, so that's not something the data warehouse is doing for us.
I agree with your experience with QuickSight, it is exactly my experience. My preference is to continue using the reports we generate in the app, but I'm trying to wrap my head around cases where this ends up being the better direction.