Please keep your answer spoiler free, I am curious if there is any lock-out content when you progress a certain amount. For example can you get locked out of side quests and content by progressing through the main storyline first? I'm sort of burned out on the idea of being a completionist.
Heart of Stone has a single main quest line. There are some different relatively minor options you can make in the main quest, but the side quests are unrelated and will not disappear.
As a light warning, during the wedding party segment of the main quest line, there are some optional activities which will become inaccessible if you progress the main objective. These activities are mostly humourous scenes more than anything else.
Blood and Wine covers a large new map structured similarly to Velen. There are a great many side quests, and I can't promise you the main quest's completion won't lock any out, however it would be few if any.
The very end of this DLC's main quest has two completely different paths to the finale, and three possible endings from there. There is a point of no return warning before this begins, however unrelated quests are still available after the finale.
Original answer, from the perspective of the core game's main quest:
As far as I know there is nothing you'd miss by beating the main quest before starting either Heart of Stone or Blood and Wine.
There may be some minor dialogue differences, but the two DLC's stand on their own. Blood and Wine in particular is in a completely new map. Both have rather high level requirements which you should reach near the end of the main game.
Nothing is locked out in the main game by completing the expansions.
Note that the expansions are tuned to your completing the main game. IIRC, Most things in them start at level 25+ in NG, and 50+ in NG+.
There is one bit in Hearts of Stone that's "locked out" if you complete the Battle for Kaer Morhen, but it's not critical - a cutscene goes slightly differently, and you're able to talk to Vesemir about it; that's all. Hopefully you'll know it when you see it, and even if you miss it, you can watch those scenes on YouTube.
It's probably fine that you're not a completionist. If you were, then completing every single quest in HoS and BaW in NG would put you somewhere around level 40 or so - overleveled by about 10 to start NG+. That said, that can be kinda fun, esp. if you're playing at Death March difficulty.