Staying at my undergrad for my PhD
I am in a very similar situation. I'm at my undergrad school for PhD. I would say that it's more important to be in a place where you feel you are going to be happy and productive. There's no point going to a new place if you're not convinced you'll be able to get a good PhD out of it. If you are productive and publish good research it won't matter where you did your undergrad. I would also suggest looking for internships in industry or applying for travel grants to visit collaborators in other institutions to get some diversity on your CV. Hope that helps!