The term "point of no return" is kinda tricky here. In most games it means "if you go past this point the game is basically over and it'll only ever return you to that save point".
You can continue to play the game after you beat the final mission. But the Isle Of Mists part will make you auto-fail a few quests if you have them open. That's what they mean by "point of no return".
As far as the DLC....I think the suggested level is 30 so you're straight. But as far as
when you should play them....
"Hearts Of Stone" adds a cool bit of dialogue to the final scene if you're still on the main story...."Blood And Wine"
should be played after the main story is over. Trust me. You can technically play it at any point but it's the real end of the story.
You'll see what I mean when you beat it.
Fred.