I have two Questions concerning "Hearts of Stone" and "Blood & Wine":

1.) I have just finished TW2 a month ago and will start TW3 in the summer.
Is it better to start TW3 vanilla (which means buying the Expansion after the playthrough)
because the expansions will just slow down finishing the main game significantly?

2.) I have the boxed version, do I have to buy the Expansion pass because the two
Expansions as a standalone product are not compatible with the boxed version?

Best regards

After you install box version,get it updated then the expansions will be compatible then decide on the expansions.

As you have the boxed version, I would use the code from your unit to redeem on GOG. Then download the updated game, latest patch, DLC package and expansion pass DLC's from your GOG account. Install in that order. The boxed game is typically the original released version.

I wouldn't worry about HoS or B+W "slowing down" your finishing of the game. You will be into this game for many hundreds of hours at any rate.

Start at the beginning - and you will find that you will probably finish - or almost finish the main game before you are even able to start the expansions, due to their level requirements. It will be your choice - considering you are at a high enough level - if you want to leave the main quest and do HoS and B+W and then go back and finish the main quest - or finish it first and then do the expansions.

I finished the game on my first run and then did HoS. Then I did a "completionist" run in a NG+ and found that I had leveled up enough to be able to veer off and do HoS prior to going to Act 3 in the Isle of Mists. I will do B+W when it is released - and then go back and finish the main game.

It's all your choice, really. But I suggest just redeeming on GOG for ease of installation and the latest updates.

Galaxy? I don't use it and I suggest you don't unless you really need/want those features.

https://www.gog.com/support/the_witcher_3_wild_hunt/bdownloading_redeeming_and_installing_the_witcher_3_faqb#FIND_KEY