For our DBs (which are often unsharded), we've found the best performance using the user account ID as the first part of the cluster key and then a sequential id for whatever the record is as the second.

It's not as good as just a sequential ID at keeping the fragmentation and data movement down. However, it does ultimately lead to the best write performance for us because the user data ends up likely still appending to an empty page. It allows for more concurrent writes to the same table because they aren't all fighting over that end page.

UUIDv4 is madness.