link diversity is a thing, so you might want to build up as diverse of a link profile as possible. That's one thing...
Another thing, google are looking for natural growth of backlinks, or organic growth if you prefer to call it that. That is, consistent growth of links, not 2 links a day for 3 months and then stop. Natural link building, FWIW, means CONSTANTLY building links for as long as your site exists.
Sure, at some point (after you achieve the desired rankings) you can slow down, but if you fully stop acquiring links and / or publishing new content / updating the existing content google - in their retardness - will think that your site is dead and they'll start de-ranking you slowly (at least it used to be like this a few years ago, I'm not sure if it's still the same today, but it's one of the things you need to consider). And that's the 2nd thing...
The 3rd thing: link velocity and anchor texts. You might pull the previous 2 conditions perfectly, but if your link velocity (as deemed by google) and anchor texts are not safe, you're going to stagnate / get penalized.
This guide that's been created by a true expert in link building (
@tiiberius, of course) will help you understand where you fall / fell short, so give it a read and you might be able to figure out what you've done / are doing wrong:
https://www.blackhatworld.com/seo/guide-how-many-links-per-day-week-or-month-should-i-build.1502403/
Last but not least, the fact that you're stuck on a fixed position on pages 2-3 tells me that you're not necessarily doing things wrong, but - and I know you hate hearing this, but I'm going to say it anyway - this could really mean that you need stronger links, ie. PBN links. It sucks, I know (because good PBNs are expensive), but they're expensive because they have power, and if you're in a tough niche or are trying to rank for tough keywords (buyer keywords in profitable niches, that is) - and my nose tells me that this is the case - then you're going to need PBNs to get better rankings...