My little business uses PG. Had a unique need for PK IDs. It was pretty easy to make a little extension in C to generate them for us, and linking in PG was super easy (but the .so has to be on replica before loading) and we only used the TEXT type cause I didn't want to figure out making a new 128bit data-type. But can't do this in RDS.
Interesting, sounds (without more detail) like uuid datatype / uuid-ossp (which is available on rds)?
ULID - https://github.com/ulid/spec it's like 48bits of control and 80 bits of power packed into 128bits of fun!!