Folks might say "chard" but I haven't grown it in many years. Instead, when I can find the seed, I've grown perpetual spinach.
It isn't a spinach either but rather, a type of chard. How is it different? It doesn't have that heavy stem. It tastes more like a beet to me, which is another possibility for you but it doesn't do too well as a green over very many weeks of summer for me.
Escarole that I had for the first time last year, did fairly well into the first part of summer. Yeah, it's supposed to join all those other greens during springtime but I'd like to push that season envelope, too

!
Supposedly, a large leaf kailaan was gonna help me do it this year ... I don't know what I did wrong - it's already bolted

! Several years ago, that variety (South Seas) did quite well into summer.
Portuguese kale - that might be my best suggestion. After several years of growing it, I'm just about to declare it a winner! However, I've got it's sister, Scotch kale, right through the summer, too. What about their cousin, collards? Are you
not a Georgia collards fan, 'Dorie?
For that matter, what about the other cousins, cabbage? I've got quite a bit of Savoy cabbage, DW's favorite, well into summer.
Steve
Keep in mind, it's hot and dry here during the summer - not hot and humid. Not perfect "greens country" but a bit of contrast from much of the country.