Yeah if index + count(*) works fine, then that is enough. This will usually not be a purely index-only scan though in practice, in MVCC databases. Since it sometimes has to inspect the actual row to see if its visible to the current transaction or not. So it's "mostly" an index scan. If you're not getting an index-only scan, count(*) can become a bottleneck in my experience even with FK indexes.

[All just my understanding]