This is an amazing guide man!
Often I come across customers who need to understand KW research better. In past I had people who wanted to rank for KWs as hard as (the next ones will be similar to theirs, not their KWs, for privacy): "Best tablets".
I remember I told them KW is hard, chances are near 0 for anyone with a new website to rank for this, and the reply I got back was: "But I am willing to work day an night"/
This remembered me of an old friend who always told me: "work smart, not hard". Best advice ever.
In my honest opinion KW research is the basis of a website. If you got bad KWs, then you are on a bad path and you are going to waste a lot of money. If you have the right KWs, then you will save a lot on the off-page, mostly because the less competition it is, the easier it is to rank, so, the cheaper it is.
After you have good KWs, you need good on-page optimization, without that you are just going to spend a shit ton on links just to make google understand what your page is about. Thing that you could do with proper on-page and no off-page.
Only after the above are met (good KWs, good on-page) then you can think about off-page.
And always start with pillow links, NOT with PBNs. And please trust me on this, I am a PBN links seller, and I advise you never to start directly with PBNs as you will waste $$, my advice should be worth something if you keep in mind that selling PBN links is my job and you would think I want to push people to buy. Don't send those to a newly built site with 0 Backlinks, you will just waste your money (not because they have no effect but because the rapport $$/effect is low + high risk, as it looks unnatural".
PBNs imo are not to push a KW from N/A --> 30. that's the pillow links and on-page job. Use PBNs to push 30-->5 or 8-->1.
I will finish with a similar but different advice than my old friend's: "Invest smart, not hard". First thing you have to do on a website is: put your time into KW research, on-page, build diversity links, wait.
That's all I had to add

Have a good one guys!
Cheers !