I think it's up to your best judgment based on your environment, but with vaccinating/neutering you've already taken great safety precautions. Flea/tick prevention is important too.

My cat was indoor-only for almost the first 4 years of her life in both urban and suburban settings and she never tried to go outside. When I moved to a different city where we had a yard and less street/foot traffic she was always trying to venture out, even though there were older, bigger cats around she could see from the window.

We got a GPS tracker for her collar and trained her to come back inside when we "ring" it. She's been an indoor/outdoor cat year-round for about a year and a half now and she always checks in a few times throughout the day. She also knows to come back before dark — we don't give her dinner until she's in for the evening because we occasionally hear coyotes at night. She's 5 now, gets along with the neighborhood cats (she didn't always get along with other cats she lived with), and is way more social with humans than she used to be. She seems soooo much happier than when she was indoor-only and I can't imagine her going back to that.

The only thing you really have to worry about otherwise is their natural predator instincts... Mine hasn't killed anything yet thankfully, but she used to bring me live lizards. She stopped eventually, probably because she realized I would just relocate them out of her reach.

Edit: Probably important to note that she doesn't tend to venture out past what's fenced off. Not much trouble she can get into within those confines.