How do you organise your contacts list on your phone?

I have nowhere near the number of contacts you do but something that helped was having a section in my notes app dedicated to them where I write as many details as possible, like if they are a friend, relatives, coworker, how we met, their birthday, favourite color/food/drink, eye color, etc. Think of it as a personal CMS or a CRM. Your phone's contacts app might not be a good fit for such a system, for example I'm using orgzly for this, and I sync it to my laptop for backup and easier editing. Also, the contacts app and orgzly are not connected/syncronized in any automatic way, from time to time I take a look in both and see which contacts are still relevant or not and adjust. For example there is probably no reason to keep around the phone number and notes of the Airbnb host from my vacation 6 months ago so I remove that.

As advice for how you can manage your 2k+ contacts I suggest you use a similar app (in case you don't want orgzly for some reason) and just type in their name, contact info (phone, email, social media) and a one sentence fact about them. Let's say do 5 a day, and group them by relationship type: family, friends, coworkers, teachers, etc. Then try to remember as much about them as possible, even follow up with some you don't remember and ask some questions, like "Hello, I have you in my contacts list but I don't remember how we met, sorry about this, but can you refresh my memory? Should we still keep in touch?". Feel free to customize that as much as you want. In the end you'll probably end up with just a few hundred that you really want to keep around.