I played through the main quest once and barely started on the 'new plus mode' or what it was called, bought the expansion packs, but I guess I need to get through the starting quests to get them..? Is it better to load the endgame instead? I'm starting some early side quests again and all Gwent cards are gone...

Thanks.

Thanks. I've learned that it might be better to load a late game save of my first playthrough rather than an early from the 'New Plus game' mode, unless I want to level my character to level 60'ish...

It depends on if you plan to play through the entire game again on NG+.

If you just want to play the new content in the HoS and B&W expansions, simply load your endgame save from your first playthrough and then start HoS by picking up the notice from the 7Cats Inn. When finished HoS then start B&W. You can play them both simultaneously if desired, but B&W is a higher level than HoS - and it makes some sense to finish one story completely before going to the next.

After you have finished the game and played both expansions - you can then start a NG+ and have the advantage of most of the extras that the expansions offered for your NG+ playthrough.

Really, though - it is totally up to you on how you want to do your playthroughs. Both of the expansion stories are quite different from the main playthrough story - and are very interesting and well done, so you may want to play them now that you've finished the main game.