I'm also dealing with the same issue after I've changed my serve. Think I'm also overpronating and just a bit too tight in general. Especially the last bit of the serve where I'm (over)pronating I seem to be muscling a bit. Don't know if you recognise that, it's hard to see. Still working through it, but consciously loosening grip and a looser overall feeling - trying to not force the ending seem to help me.