Insulation in every interior wall and 5/8” Sheetrock throughout makes a custom home feel and sound like a custom home.
Every custom with uninsulated 1/2” rock on the walls, I make the comment the walls remind me of low income housing, because they do and are no better. Often the installation labor cost of 5/8” rock is no different from 1/2”, the added material cost is reasonable and it changes the feel of the house. It requires door and window jams to be sized for it, but that’s common.
Unfaced fiberglass bats are dirt cheap and labor is minimal if the insulation contractor is used to doing it. Rock wool or wet blown cellulose is a much better sound insulator than fiberglass, but more expensive.
I’ve been a gc in nicer homes and spent most of my years as a lead finish carpenter in nicer homes and remodels so I’m a little biased, but nice trim installed and painted well really makes a house pop. Perfect door reveals, casing reveals that are consistent, moderately wide case and base installed straight and inline with the walls, scribed base to the floor, etc.
Solid core doors are standard in nicer houses, but there are often 3 different weights available.