I‘m expected to submit my Phd in few months.
I passed my upgrade with criticism about my work and I discussed that with my sup and she refused to admit that there is any flaw with my research design. I considered all the boards comments and changed my design and my sup never comment about that

During my phd I raised my concerns to the uni saying that I am not receiving enough guidance and they said they’d resolve this informally but nothing has changed . I became convinced that there are 2 options either to change supervisor or remain silent
I completed my thesis by myself without any guidance and my supervisor would read occasionally and gave me very positive feedbacks but I can’t believe her

Recently, I learned that my supervisor first phd student failed her Viva R&R. I tried to contact the student but she never replied. So I emailed my supervisor telling her that I am extremely worried and I can not afford to fail.

I told my supervisor that I need someone to revise my work as I am not confident about it and I had my first draft ready by that time and my supervisor sent the draft to her colleague who took a look at my thesis but have not actually read it and her reply was that the thesis looks orginal but the argument is not well articulated and she is not sure how the thesis is different form other works. My supervisor told me that these are very positive comments and all what I need is to make more emphasis. I don’t really see it in a positive way and I feel my thesis is horrible and the aim of my study is not clear

My question is what is the wisest thing to do now: shall I remain silent and submit or shall I raise the issue to the department and demand that someone read my thesis in details and if the suggestion are major I could apply for an extension

Never expect the uni to help you when you are in disagreement with your supervisor!
I've seen so many cases! Whenever the student has a conflict with the supervisor, the uni always protects the supervisor but never listens to the student! Even when the supervisor is an arsehole! Even when the uni gets lots of tuition fee from the student!
You can try and raise the problem to the department or the Faculty, but the result will probably the same - they just blindly side with your supervisor and provide you with no support.
If you can, you'd better ask someone in your research area for some advice. If you can't do that, please believe in yourself.

Yep, that is true. I think I'd want someone different to read it and get their take. To try and give you more confidence and perhaps give some more feedback you can address to improve it.

Re the comments the colleague made: those are actually encouraging although I understand you being worried in the context of what has happened with the previous student. It sounds like she thinks it has what it takes to be a thesis and add a little something to the field, but it needs a bit of work to make it clearer what was known before and what your thesis adds.

My thesis was absolutely horrible and I hated it too. I didn't feel like it added much. But perfecting the writing and framing things well makes a difference and can make you feel more confident about what you've done. For me, it always helps with any project to write an abstract and perfect that. Once I have a convincing story or case with my abstract, I feel like it is what it is, not perfect but it makes sense and adds something. You could try this technique to help clarify your research aims etc.

I learned that my supervisor first phd student failed her Viva R&R. I tried to contact the student but she never replied. So I emailed my supervisor telling her that I am extremely worried and I can not afford to fail.

Hi PhD2020, I had an almost similar case but it was with my MSc supervisor, who was really horrible towards me and I brought it up to the department, but they did not make much effort to take actions towards the supervisor. I went to the Student Union and they were able to take my case better and support me. Do you have similar organization at your University that may help you? Maybe.

Update
I became the second 😭

I'm sorry to hear this. What did they say? Was it the documentation quality? Or research itself?

They said I need to rewrite my findings and discussion to make my potential contributions to knowledge absolutely clear to the reader. As though rewriting the findings would result in a different contribution . The internal examiner was an issue here and the viva was not really fair but what option do I have other than do the corrections and hope for the best. The months before the submission were very stressful and the viva went exactly how I imagined. A horrible day and I would remember that day as the viva attack day . I never thought that examiners would behave that way amid of pandemic and my supervisor never contacted me except sending one email to threaten me. I really lost confidence in the system and not sure should I appeal, quit or just spend another unfunded year on my boring phd and try to resubmit

You DID NOT fail. You got major corrections. You have not failed and you have every opportunity to get your doctorate.
You need to be careful with how you are wording things. Your supervisor has not threatened you, they have responded to your aggressive emails as you wrote.
Rewriting your findings doesn't change the contributions you've made, it makes it clearer to the reader. Again, it's a common correction; we write it because we know it, to any other person the only basis they have for understanding is the words we write