I think you’re thinking out the cache property of a sequence, but it defaults to 1 (not generating ranges at once). However, Postgres only needs a lightweight lock on the sequence object, since it’s separate from the table itself.

MySQL does need a special kind of table-level lock for its auto-incrementing values, but it has fairly sophisticated logic as of 8.0 as to when and how that lock is taken. IME, you’ll probably hit some other bottleneck before you experience auto-inc lock contention.