I have two experiences of this OP, so I hope that talking about them both will be of some use to you.
First time around with XH, we discussed marriage between us, decided that he would propose somewhere sometime as a surprise, but then every weekend away, every holiday, meal out etc I was on tenterhooks waiting for the proposal, but he hadn't even bought a ring. It was just a series of disappointments for me.
I ended up telling him that I'd been waiting and he had thought he would do it in a year or two, obviously no rush! In the end I didn't get an engagement ring, we got married pretty quickly anyway and it was all very non-traditional, especially as he'd been married previously, so I wanted it to be very different from his first time.
So we're now 6 years post-divorce and I have a new partner of 5 years. We have also discussed marriage, but he is more traditional and thinks he should have been the one to just propose out of the blue. However, this would not have been for 5/10 years as we both have children so this is not just some romantic whim, it's a massive undertaking involving lots of people!
I lost my mum in the intervening years and now have her engagement ring, which also belonged to my nan, so I would like to have that resized and be able to wear that as my engagement ring. It means finding a jeweller who can do the work on this ring, being there to get measured etc and it means I know exactly what the ring looks like! However, it will have more meaning to me than one DP has chosen because it fits in his De Beers advert price bracket. We also have quite different tastes and I would worry that I end up with a ring I don't like!
I sent him a text with a photo of my mum's ring saying that I'd like to be able to use that and he now jokes that this was me proposing to him!
In short, I think the romantic surprise proposal is a bit one-sided - it doesn't take into account that the woman has any say in when or how it happens, when this is a major life-changing event and should be something which is discussed in great detail. The fact that you've already talked about it, rather than waiting for him to be ready, shows a partnership of equals. Why not continue in that vein and make your official engagement a joint thing too?