The Rasticore stuff started late Season 2. I know she had him try to kill Star and Marco earlier that season, but she wasn't practically fucking his arm his first appearance. And yeah I agree with you.
Or even I can believe they planned for Heinous to be related to the still-unrevealed character with spades from her first appearance, since Star and Moon have the red card suits and are mother and daughter and so having the same relation for the characters with the black suits actually makes a lot of sense. But I don't think they had anything about Eclipsa beyond "there will eventually be a character with spade cheeks" planned in S1.
Heinous never really came off as being a monster until S3 when they made her one. Like even when her cheek marks reappear in St. Olga's it's treated like that's it and that's all she's hiding, not that she has much freakier biological stuff going on too. They also have her just using makeup in S2 even though in S3 it seems like she can shift her own appearance, but then they also mention the makeup in S3? I don't get it.
I feel she was always planned to be related to Eclipsa, but the monster stuff was more of a last-minute thing, like Star's getting into monster rights now so we need someone to be a monster too. For sure they had it in mind as soon as Into the Wand, I was even one of those anons who theorized then Heinous could be Eclipsa's daughter because she had some features of Globgor, but they still did kind of a shitty job hinting at that in the actual show because it was mostly a crack theory until Monster Bash.