I believe you are on the right track tying the idea to “opportunity.” This is reinforced by Charles’ words when leaving Brideshead after being accused of wanton cruelty by Lady Marchmain:

I felt I was leaving part of myself behind, and that wherever I went afterwards I should feel the lack of it . . .

His “opportunities” for moving up in the world have been foreclosed (albeit temporarily). He even makes reference to buried treasures which could be construed as a backward reference to the day spent with Sebastian eating strawberries and drinking champagne and smoking Turkish cigarettes, which was the day of his first visit to Brideshead. All other visits thereafter were revisits. The question remains: is the last visit he tells us of truly the last re-visit?

I went there uncertainly, for it was foreign ground and there was a tiny, priggish, warning voice in my ear which in the tones of Collins told me it was seemly to hold back. But I was in search of love in those days, and I went full of curiosity and the faint unrecognized apprehension that here, at last, I should find that low door in the wall, which others, I knew, had found before me, which opened on an enclosed and enchanted garden, which was somewhere, not overlooked by any window, in the heart of that grey city.

This “low” door is not so obvious and not as traveled. I like the phrase, “not overlooked by any window,” as if to say you can go there privately without anyone knowing the path you’ve selected.

I also corrected the Category link to reflect a tie to Brideshead for us.