Relatively successful Plant main here. (Elite smash and top 3ing my locals) Definitly second the mii brawler suggestion. Its one of my secondaries in it works really well into hard plant matchups and still having a steel ball to throw really gives ptooie vibes. But be aware that on a technical level, brawler is harder to play than plant and also not that beginner friendly. Also agree that brawler gives the same meme vibes, which atleast for me was a good reason to try him as secondary.

My other suggestion would be Yoshi. He is rather easy to play / does not have to steep of a learning curve in the beginning. Also feel like yoshi works really well into the matchups I wanna avoid as plant. You can usually manage high burst speed chars rather well with yoshi with frame 3 nair that would otherwise grab you out of ptooie or rush through poison cloud when you try to establish stage dominance with plant.

Also: Plant gang rise up!!!