I 100%'d everything in Paper Jam. Post game/completion definitely leaves you wanting more, but ride there (read: ride, not grind) was definitely enjoyable.
I preferred going intentionally underleveled/stocked to enjoy the most out of the experience (balance-wise, the game becomes way too easy to break. Even if you don't go for optimal Rank boosts/perks, use stat boosting beans, and opt out of the Assist/easy modes, the battle card system/amiibos and the gear you can buy can let you cruise through the entire game, which only becomes exacerbated later on in the game due to enemies dropping rare item drops more frequently). Up until the optional/secret boss, I didn't bother grinding/forming specific sets of battle cards (literally playing Yugioh with infinite Pot of Greed cards), so when I did the (optional) Boss Rush mode, I was left feeling bittersweet knowing it's only as much of a challenge as you make it. Though I guess that's true for all games. However, out of all M&L games, it never felt quite as evident as this.
All of this is from a completionist viewpoint, and is only my opinion, and in no way should deter anyone from trying the game. Though it did leave me wanting more, I absolutely adore the charm of the game, and its easily up there with the best in both respective series in terms of writing/humor.
Admittedly while I will also be taking a break from it for now, I can easily see myself going back to replay it frequently unlike DreamTeam/StickerStar everything after Bowser's Inside Story