I think a lot of us have the same issue. Like you I have a SA at Selfridges and she is a star, she's been very frank about the fact that stock levels has been very disappointing. There's a bag that I want and I want a specific colour as well, she straight out informed me she hasn't seen it in two years she's been working there despite the fact this is not a quota bag. I do agree with you that the boutique is quite small, especially if you want to have a look at the ready to wear, the new Bond Street or Cadogan place is a better choice.
I also agree with you that although it really depends on the individual, it can be a lot more work if you do decide to shop at Cadogan exclusively. Same has been said about the NBS boutique as well. Purely based on stocks, NBS is a logical choice, I'm not exactly sure that Cadogan has much larger stocks than Harrods or Selfridges. I popped in couple of times for bags and silk items but they never had what I was looking for.
My own wishlist is about to expire in a couple of days so I will have to renew it somewhere. If I were to only think about the experience and my SA, I would choose Selfridges but because my SA had specifically advised me about the stocks, I am a little bit hesitant. I am aware that for some, they were able to receive the quota bags relatively easily and without too much wait but I would bear in mind that that was in the past and now we live in a post-covid world where the situation really is totally different, especially in the UK.
It is far-fetched but if you ever do get a chance to visit and travel to New York, I would definitely go to the boutique there. They used to cater to tourists but since the pandemic there are no tourists and somehow the stock levels have not changed (this is what I heard from my cousin who lives in NY). She put in a request for a black, gold hardware B30 just before Christmas last year and she received the bag couple of weeks later in January. I was so jealous but at least I got to see her enjoy it through our video chats hehe
Thanks for your insights ! It's funny to see how we have the same "problem" right now

What are you planning to do for your wish eventually ? (which store will you choose). That's really good to know for New York, but I think it will more be for 2022...