@davids355 could you explain "building some buffer sites to rank some long tail terms related to the main term " this please or give further infos about
Yourwebsite.com/main-keyword - you want to rank this site for main keyword.
abrandnewsite.com - you build this site and target something related to your main keyword. so if your main keyword is Ping golf clubs, then you build this new site targeting this keyword - review of ping 5 iron golf club. That related term might get only 100 searches per month, but have low competition. So you can try and rank your new site for that term. You build your pbn links to this new site in order to rank it. And you also link back to Yourwebsite.com/main-keyword - using your main keyword as anchor text. Then the juice from the pbns gets filtered to your main site.
The main objective here is to keep your main site safe and the links pointing to it in your control - if you get penalised you can remove the link from abrandnewsite.com and save your main site. Also if abrandnewsite.com gets penalised then you can throw it away, its not important.
If abrandnewsite.com ends up ranking well, then you are sending the traffic to your main site anyway, or you might end up just redirecting it if all works very well.
Others may say this is a long way of doing it, and it is, and you will lose some power from the links.
But what I am saying is this - if you are new to SEO and you dont know what you are doing, then you are risking less with this method. If you end up getting a few of these new sites penalised it doesnt matter and you are learning along the way.