What performance advantages does append-only postgres allow?
If you wish to benefit from index-only scans (IOS), you will still need to vacuum your tables even if they are append-only.  Indeed, autovacuum was never adjusted for the needs of IOS, so it is likely you will need to schedule your own vacuums for IOS to be maximally effective on append only tables.  Certainly don't disable autovac, it will naturally not do stuff when there is nothing for it to do so no good will come from disabling it.

Other than that, I don't think there is much you can do.  This situation will greatly benefit from the freeze map introduced in 9.6, but there is nothing you need to do to activate that, other than use a version at least that high.