Not really a spoiler. Master classes aren't really that better than Advanced classes, growth rates are generally worse than Advanced classes. Except for Wyvern Lord + Falcon Knight for whatever reason, they're easily the best classes in the game. Ignoring the fliers, the only thing Master classes give is a mount or double warp/siege tome in the case of Gremory. Its less about skills and leveling up and more immediate boost to unit movement.
Yeah, nah. Master Classes for the most part are way better than advanced classes. Growths are significantly less important by that stage in the game compared to utility. Having a warlock that can keep up with the rest of your team and canto away is a godsend compared to having some slightly better growth rates, which again isn't that important that late into the game. Generally there's an upgrade path for almost all advanced classes besides a few.
(bolded is preferred with maybe some exceptions)
Hero -> War Master or
Wyvern Lord
Pegasus Knight -> Falcon Knight
Wyvern Rider -> Wyvern Lord
Paladin -> Bow Knight or Dark Knight
Fortress Knight -> Great Knight or
War Master
Sniper -> Bow Knight
Warlock/Dark Bishop -> Gremory or
Dark Knight
Grappler/Warrior -> War Master
Sword Master, Assassin and maybe Bishop are the only ones without a direct Master Class.
Mortal Savant and Holy Knights are a waste of space, there's no reason a Sword Master should ever change to it with how much speed you lose. Just use a levin sword for 2 range or use an armor slayer for high def units. Holy Knight is just pointless because Physic exists and you don't get 2x uses of it as a holy knight, so there's no reason your healers need to have canto.
You
can make a Bishop a gremory, since they keep the 2x White Magic uses, but also Bishops don't really gain that much from changing. Gremory is only preferred for people who have access to great black/white magic spells. Dorothea is a solid example where she has both physic and meteor, otherwise you may as well stay bishop or go dark knight.