Currently, my traffic from Search Console is between 6k - 2,00k+, as you can see. However, the impressions are significantly higher."
This graph of clicks and impressions is pretty standard and I don’t think you have unreasonably high impressions and lower clicks (i.e. your CTR is great!). It also means that most of your content has matured and Google is placing your pages exactly on the spots where users intent to visit them.
There's competition in this niche. My competitors rank #1, while I'm at #2. Any ideas on how to overtake the top 3 rankings?
Aren’t you already in the top 3? If you want to reach the #1 position and dethrone your competitor(s), I wouldn’t change a single thing about the content (because once you are in the top 3, editing your content can even impact it negatively and lower your rankings – although this is mostly temporary)
So building good quality backlinks is the only sensible option. I wouldn’t venture into PBNs unless you know how it really works, and are willing to take the risks for a greater reward.
But you can utilize decent guest posts and niche edits in your niche (along with some “lighter” links for diversity such as comments, forum posts, social media, and other nofollow links)
If I was to give you the most important factors when choosing a potential guest post website, it would be:
1. The website should be legit (not a link farm, as mentioned above)
2. It should have significant traffic (2k+) and domain age (at least 2 years old)
3. DA/DR metrics do not portray an accurate picture of the website, as they can be easily manipulated. But you can check the DR/backlinks graph of the website in SEO tools to see whether it had a sudden jump in the authority score or is it a sustained increase (the latter shows genuine authority)
4. Lastly, it should be relevant to your niche. Or at least, you should be able to come up with a topic that is the perfect "bridge" between the linked URL and the linking URL.
One of the best ways to ensure that the site isn’t a link farm is to check whether they have revenue sources other than selling links. (It could be display ads, affiliate offers, own digital products, etc.)
Final note: Before you start buying guest posts, make sure you consider your historical “link velocity”. It is the pace at which you’ve been acquiring backlinks in the past (natural ones). Since you have 600+ pages and over 2k daily visitors, I’m sure you would have picked up some random backlinks from blog sites as well as autogenerated sites. So ideally, you should start building links at the same rate (or slightly higher) as your historical rate. This will mimic a natural link acquisition and keep you in the good books of Google.
Hope this helps!