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The value of this DLC depends on what you get out of Shiren games. If you're not someone who bothers to complete every dungeon in the game you might not touch half of its content. In fact, at first the DLC does nothing but add a plus to the title screen, until you beat the first main dungeon, at which point 7 of the extra dungeons are unlocked.
3 of the first 7 dungeons—Forest of Frugality, Training Path of Deadly Strikes, and Floral Garden—have fun and novel gimmicks and are worth playing for anyone who gets to the point of unlocking them, but not necessarily worth $13 by themselves. There's also Clash of Weapons and Shields, a dungeon where no items spawn except equipment, which can pose a challenge, but since you can bring your own items, is mainly a "pay to farm" thing.
Peachy Trap Domain and Battleground of the Rivals require beating multiple of the base game's dungeons to unlock, and they're pretty much just mashups: one is about both turning into monsters and laying your own traps for the enemies, and the other is a dungeon where the final few floors are a boss rush. Not very creative but seem like solid additions.
Do you like 99 floor dungeons? This DLC adds 4 of them! You might appreciate that most of them are playable long before you unlock the one in the base game, but the game doesn't properly explain the point of them so:
Timeworn Cave removes gimmicks of Shiren 6 that aren't in previous Shiren games. Researcher's Experimental Site goes further than that and is a recreation of Fay's Final Puzzle from Shiren 1, removing every item and monster that doesn't have an equivalent in the OG. Winding Valley is more gimmicky: you start from level 99 and level down.
Finally there's Soul of Serpentcoil Island, which is only unlocked after you beat the base game's 99-floor dungeon, and is the hardest version of it. This is actually the 3rd variant of that dungeon since the base game also unlocks a 2nd version with in-between difficulty at the same time, but you don't have to play it to unlock this one.
As for the new characters, since Sacred Tree is a way to replay dungeons you've already beaten, they may be pointless for you if you don't care about doing that. Honestly, they should have also let you play as them while rescuing, that would have made it a lot better.