I’ve grown leeks last few years in 6b immediately adjacent to 6a. (Doing kohlrabi this year instead). They grew just fine for me. The only tending needed was weeding. I start them indoors mainly because have tomatoes and other things to start as well. Then I don’t have to worry about germination rates and thinning either. I just pop them in the ground in the spring, let them go all summer, and harvest them throughout fall as I need them. I’ve never had a problem with too hot, too cold, or flowering/woodiness, though I’m sure there is variation between different cultivars. I grew American Flag leeks.

I tried to grow leeks in my garden, but I don't think I did it right because when I went to harvest them the stalk was not tasty like it had become woody or something. Do you need to harvest them before they start to create the flower?

Never grown them but I am a big leak fan. I feel your enthusiasm

https://leekspin.co/

I've grown leaks in SE MI, which was recently reclassified as 6a, as a spring into summer crop from seeds started indoors. I didn't have any problems with health or yeild. Haven't tried them as a fall into winter crop.

Like all things produce, "Leak" is a family with different members being more cold tolerant than others. It looks like 20 °F is the floor, but dropping below freezing can do damage.