I too need to do percentiles. One option is loop through the grids but bin/histogram it. You'll get a really good 99% from a 1 Kelvin bin width.
Also, I've found the diurnal profile from ERA5 analysis can be abysmal in some locations. ERA5-Land is much better, high resolution, though only available over... er... land.
To your point about not relying on cloud. Noted in the Google option [1] link above:
> Update Frequency: The ERA5 dataset is currently not refreshed in the Google Cloud Public Dataset Program. The program provides ERA5 data spanning from 1940 to May 2023.
Another alternative, Amazon [2], also deprecated:
> The provider of this dataset will no longer maintain this dataset. We are open to talking with anyone else who might be willing to provide this dataset to the community.