Don't expose primary keys to the public. Create separate external ids instead. I personally use BIGINT primary keys, with UUIDv4 external ids, but any random string will do.
Certainly seems the easiest solution. A lot of handwringing about poor database performance of UUID4 when you could use it exclusively for an external identifier all for the cost of an additional column.
Also, poor performance of UUIDv4 primary keys is most related to how write heavy your table is in the first place, and in particular how insertion heavy it is. In theory your users table shouldn't be very write-heavy, even if it may be insertion-heavy compared to other writes.
