Not all stats you mentioned, but at least some. I mentioned those as a workaround as I don't think that these stats would ever be a priority feature. Sorry if it was not helpful.
Thank you very much for the explanation.
The only good solution for scaling I know is YDB, but it requires more hardware than MySQL and a different SQL dialect.
I thought for a single query you don't need a cross join as it is possible to filter/index by user. But I agree that it is more computationally intensive than a filter on a single table. Especially with this number of queries.