I have the stats_temp_directory in a tmpfs mount. I do have pg_wal on the same premium SSD storage volume as the data directory. Normally I would know to separate these but I was told with the cloud storage that it's all virtualized anyway, plus storage IOPS are determined by disk size so having a smaller volume just for pg_wal would hurt me in this case. The kind folks in the PG community Slack suggested just having one large premium cloud storage mount for
the data directory and leave pg_wal inside because this virtualization removes any guarantee of true separation.
It is true that the IO is virtualized but that does not mean that separate volumes won't help. In cloud storage you are granted specific IOPS/MB/s per volume. Separating pg_wal to a new volume mount will take pressure off of page writes and allow the wal to write within its own prioritization.