Years from now the two of you will retell the proposal constantly and the ring's specifications never. That ratio is the entire planning brief: the moment is the product, and everything in this issue up to here was set dressing.
Surprise the moment, not the decision
The happiest pattern in modern proposals is an open secret: the couple has discussed marriage, timing, even rings, and only the staging of the question remains unknown. Total ambush proposals succeed mainly in fiction. If you have not yet had the conversation about whether, that conversation is the next step, not the jeweler.
Build it from her, not from the internet
Public spectacle delights some people and mortifies others, and you already know which one you love. The reliable raw materials are specific to the two of you: the place something began, the walk she always chooses, the meal she orders every time. Grand is optional. Specific is mandatory.
A photographer hidden behind a hedge is either the best decision of the month or the worst of the year. You know which partner you have.
The unglamorous checklist
Someone should know where you both are. The ring rides in the pocket least likely to be patted theatrically. Weather gets a contingency, kneeling gets dry ground, and the speech gets three rehearsals and then permission to collapse into whatever you actually manage to say. It will be the right speech. It always is.
The short of it: settle the decision together beforehand, stage the surprise around something specific to her, and let the speech be imperfect.