Let's start with some basics.
Quality Rater Guidelines
Make sure you read Google's own Quality Raters Guidelines QRG This is an invaluable resource on how Google spot checks pages for quality. Google has 1,000's of spot checkers around the world checking the quality of pages and sites using these guidelines. This is far more valuable than the hundreds of SEO "experts" selling you something.
Expertness, Authority and Trustworthiness
I would recommend you research what is commonly referred to as E.A.T. Marie Haynes has some good information on EAT and how it relates to the Google Quality Rater's Guidelines. However, you can always use the QRG docs to better understand E.A.T.
It will be very hard for a new website to outrank an older website for highly competitive keywords. It's not impossible, but your competition has the advantage of having already been establishing their E.A.T. before you launched your website.
Google "Tastes" your web pages
Google will periodically move your new pages to the top of the SERPs to "taste" if your content appeals to users and answers questions. How users respond to those taste tests will impact whether or not your page stays towards the top, or gets pushed back down.
SEO is fluid, but always wants to answer the users question
I've seen websites with what I perceive as having low E.A.T. still rank well, over sites that do have good E.A.T. However, it doesn't seem to last very long. So, when you rank well, don't assume it's because you finally mastered SEO. It could simply be that Google has been tasting your website.
Lastly, Google's algorithms have hundreds of ranking factors that are being tweaked every single day. You may see a change in your ranking, that lasts a couple of months and then returns to where it was before. It's not unheard of for Google to say, "that idea didn't work, we'll remove it." Or they could keep it and change it some more. All that is to say that SEO is not some secret formula that says "Do X and you'll automatically outrank all of your competition". Google's ultimate goal is to provide the best possible answer to the user's query - as determined by the user's response, not our own opinion of our own content.