Never an easy one to answer this, as so much depends on the individual cat. Generally speaking, most cats who've experienced the outdoors, want more of it. It's their natural state and informs much of their senses about their surroundings and their life. Having said that, there are cats who are perfectly happy being indoor only cats because they know no different.
It's understandable you're going to be worried and concerned about her wellbeing - it's a dilemma that faces all of us in this situation. There are no guarantees whether you live in the city or a rural area - cats can and do get killed on roads every day, and evidence appears to suggest that those cats who live in towns and cities may be more street savvy and ultimately fare proportionally better.
Personally, I dont know. We live in a semi rural area, but I've lost two of my own cats to accidents in 2 years, and this year alone one of our neighbours cats was killed and the other injured in an RTA.
As Sootyca says, you have to weigh up whether your happiness outweighs their happiness, or whether a cat proofed garden is a compromise. You've tried the compromise, yet your lovely girl is still curious and maybe wants a wider world than she has at the moment.
You seem to be doing and providing everything you possibly can to ensure she has a fulfilled and stimulating home arena, so I dont think her desire to explore furtehr iis from any lack of care at home.
If I had the opportunity, I'd cat proof our garden, but as it's very small, I suspect I'd be faced with two very unhappy cats, and therefore at present, I go along with my husband who also feels it's better that they live a happy and fulfilled life, for however long or short a period of time that may be. But, if that decision was the right one for you, you have to be steeled against the possibility that the world is filled with obstacles and danger.
That said, you wouldnt keep a child cooped up on the basis of "what if this happened..." and a cat is essentially a much wilder spirit than a child and with a modicum of survival instinct that is arguably more finely honed.
If you decide to bite the bullet and let her out, do so when she's not eaten, so hopefully hunger will eventually turn her towards home sooner rather than later. Good luck with whatever you decide.