You are already in at U.Mass with a good scholarship. I do not think that it is worth taking on significant debt to attend CMU instead. You also have great options in-state in California and I am guessing from your post that you have not yet heard back from some of them.
I do understand that CMU is excellent for CS.
However, I have worked for years in high tech. I know quite a few computer science graduates from MIT and Stanford (as strong as CMU) and also graduates from U.Mass Amherst. We work together as equals and no one cares where anyone graduated from.
There is a story that I will repeat from a year ago because it is relevant: I am currently mostly retired but do a bit of consulting. I consult to a company I worked at for 20 years and therefore know a lot of people there. I was given a VERY hard problem to solve, looked in detail and thought “this is tough”. I went to one of the very top experts in the world in this very detailed specific area, who is an MIT graduate (as am I) who I have known for 40 years. His immediate response: “You have to talk to X”. I went to X. He answered the question very well, we discussed related details, and we went to lunch. At lunch we started talking about where our kids were going or would go to university. It turns out that X is a U.Mass graduate. This does not stop him from being the guy that an MIT graduate goes to for advice on something that is really hard. His boss (who is excellent) got his master’s at U.Mass.
CMU is great. I would not take on significant additional debt to go there if your options are U.Mass Amherst with significant scholarship and some of the public universities in California with in-state tuition.