We live in letchworth garden city.
It ticks most of the boxes you mention.
Re schools - there are 2 state secondaries. Highfield and fearnhill. Highfield as far as I know is a very good school and fearnhill is getting better. Living in letchworth, you can also apply to hitchin secondaries, but they are usually oversubscribed. ( but it isn't impossible to go to hitchin secondaries - my neighbour's daughter goes to hitchin girls school ).
Primaries- some are good, some are ok and a few I wouldn't recommend ( like most towns). There are no awful primaries in letchworth.
Oh and there are 2 private schools. St Christopher's and st francis.( I have no experience of these )!
The station is central to the town and has some good fast links to kings cross.
Being a garden city there are lots of green areas. We have a lovely big common and a great splash park. It is a town but is very leafy. Lots of lovely residential areas. We also have a great outdoor pool, and a fab leisure centre in the town. Lots of parks.
There is a lovely Art Deco cinema and lots of things going on in the town for families. Especially in the summer. Historically the town has been a bit in the dark ages as it run by the heritage foundation. Recently ( in the past couple of years ) it has improved. There are enough shops for general needs. More nice coffee bars and restaurants are popping up so it is improving. Hitchin is a more lively town centre with a bar and restaurant at every turn..but then if I fancy that..I just pop to hitchin..it's 2 miles down the road or 1 short stop on the train. We also have a great big sainsburys.
Value for money housing stock - that's a difficult one as it depends on perspective / budget. A typical 1930s semi in a nice quiet area would be between 350 and 450k. If you don't mind a 'project' you can pick one up for around 320k. It is cheaper than hitchin in general but there are also a few roads in letchworth where houses are huge and command high prices.
We have our 'not so nice areas' like any town too.
I think it is a lovely friendly place. I have a little toddler and I find mums are nice, laid back and generally easy going people. I'm never stuck for things to do.
Just my view. You may find some people say letchworth has no soul. But I don't find that at all.
I'm really surprised you weren't keen on hitchin..most people I know ( me included ) love hitchin but each to their own! Good luck with you search ??