Then tell it you don’t like those channels and thumbs down the short stuff you don’t want, and thumbs up the long stuff you do.
Remove the “shorts” section entirely (I know you can with vanced, forget about vanilla) if those are the 30 sec videos you mean. I hate them and ignore them entirely. Simple.
It’s really not hard to get YouTube to be a pretty damn good recommendation engine. You just have to give it some constraints.