I lived in Winchester for two years in 2014/6. My then teenage Dd went to the big sixth form college Peter Simmons and she had a really great time.
We lived walking distance to the town. It has a massive one way system to get around the town centre and it can be a mare, but is probably no worst than most.
Im obviously older than you but I liked it. It felt very safe and I would have no problems walking alone at night. It had plenty of really nice restaurants and pubs and bars and feels nice and buzzy in that way.
The shops are the usual for a place this size, the Laura Ashley, M&S, etc, but the big Debenhams has closed down. It had a big pedestrian area which has a street market weekly I think.
If I were to go back I would choose to live on the outskirts with a bigger house, garden and more immediate green spaces.
It really depends what you are looking for. Like you availability to family would be high on my list. It is a ‘nice’ place, so I’m guessing the schools would be reasonable. I would say it’s similar to Salisbury.
I know nothing about mum groups, but I think it would be a nice, safe place to raise children. And don’t forget you can always move again after a few years if you hate it. House prices are expensive in Winchester, which sort of tells you that people like it for a reason.
Let’s just say it’s definitely not a “yobby” place.