Hi!
I would suggest to start a relationship with your SA. I started to purchase small items like perfume, a bow tie, Calvi, Bastia or Ulysse to begin with. After that it didn't take a long before I was offered a SO and quota bag ☺
Now I have a really good relationship with several SAs in my local boutique and I have been offered quota bags on several occasions.
So to summarize I made around 5 purchases before I got offered my first Birkin and after that I have just established a good relationship with the SAs by being nice and when I go to my local boutique I ask for the specific SAs and when I meet them I ask how they are, what's new and just being nice in general. I feel like they appreciate someone being generally nice to them helps. At least it has helped me in several of the brand boutiques like
Prada, Céline, LV, Chanel and
Gucci because now when I come back they always ask me about my family, marriage etc (with some the SAs I have even start to hang out with outside of the boutique)
I think the most important thing is to be genuine and take time to give them as much attention as they give you. Just be yourself and nice and it will get you far