If I enjoy a game, I'll go for 100%. That isn't just for Pokemon, it's a blanket statement for everything I've played. For most Pokemon games, I'll go through the post-game and finish up when it feels like I've done everything, and I've made a few full pokedexes and have a living dex. For Platinum specifically, I went back and got all the Battle Frontier prints, maxed out my trainer card, bought every item in the villa, all this bonus stuff that was definitely not necessary, and it's just because I've enjoyed the game so much I've wanted to feel proud of having a save file that's totally maxed out.
In terms of TMs, ever since they started becoming collectable items and being multiple uses, so ever since Gen V, I've started collecting them and wouldn't stop playing until I was done. This includes having 1 copy of every TR in Gen VIII.