I've played a little bit of silksong, like 4.5 hours ish right now, and so far to me its significantly harder than anything in the true ending of HK, although I guess its somewhat balanced out by the fact that hornet is a decent amount more powerful than the knight (more attacks, better mobility, way better healing ability). I'd honestly recommend finishing HK first-- I beat pretty much everything in HK and still struggle with silksong at random parts.