Native Logical Replication Initial Import Qs

Good afternoon.

I'm looking at having to move a fleet of PG 12 databases from Ubuntu 18.04
to Ubuntu 22.04. This means crossing the dreaded libc collation change, so
we're looking to have to migrate via pg_dump/restore or logical replication
for the bigger/busier ones. We're also planning to use PG 15 on the
destination (Ubuntu 22.04) side to kill two birds with one stone, as much
as I'd prefer to have minimal moving parts.

On the logical replication front, the concern is with the initial data
import that happens when the subscription is created (by default). I know
that you can tell the subscription to not copy data and instead use pg_dump
and a replication slot snapshot to achieve this manually. However I'm
unable to explain (to myself) why this is better than just having the
subscription do it upon creation. Given that I can create pub/sub sets for
individual tables for parallel operations, I'm curious what advantages
there are in using pg_dump to do this import.

I had been planning to have pg_dump pipe directly into the destination
database via psql. Is this faster than just having the subscription do the
import? I'm curious as to why or not. I know to only use the minimal
indexes required on the destination side (ie identity-related indexes) and
omit other indexes and constraints until after the data is loaded, but that
is true for either method.

Thanks,
Don.

--
Don Seiler
www.seiler.us