In this case oAuth is more because we have multiple products, might have done without it for just one, but it somehow seems to make more sense to have a separat

In this case oAuth is more because we have multiple products, might have done without it for just one, but it somehow seems to make more sense to have a separate system with isolated human data anyway, what with GDPR and all. The actual application has only the user UUIDs, can null or pseudoanonymise the references as necessary. This is B2B so slightly different rules apply for auditing etc. And yeah, obscurity is not security but is useful in itself. Smaller attack vector (can’t leak info by iteration) less competitive info leakage (you’ll never figure out how many customers we have unless we tell you). Also has advantages with regards to database replication, backup restore and sequence management, but that’s another topic.