There are basically 0 references at all to the main story, 1 line of dialogue maybe and nothing about the endgame. Triss/Yen aren't mentioned at all. You can basically treat it as a separate adventure, I think it fits fine after the base game.
It's level 32+ so if you did it during the main game it would happen right as everything is basically coming to a climax. I decided to finish the main game and then do Hearts of Stone.
The new gearset you can get in HoS is level 39 so if you did it before the main story you couldn't wear it anyway (even though I was only level 38.75 after completing HoS).
Spoilers if you do HoS before the main story (no story spoilers):
That's it. It is basically completely standalone. Since Blood and Wine takes place after the main story anyway I'd just do both DLCs after finishing it.