I finished HOS months ago and now my saves have reset to the point where i had just finished the main quest (no HOS progress is saved)

Does anything you do in HOW affect the BAW story
Is there any reason why i should not just start the BAW questline

Without HoS you will be "missing" a very useful merchant (that you can visit with HoS installed, and not doing any questline). To have him sell you stuff he will give you something to do, but you can reduce HoS to the visit of this merchant only.
Other then that - depends: if you have played any other versions of the witcher before (W1/W2) you will meet someone you already know in HoS. But for this you actually must have started the main quest in HoS.

In general B&W is kind of stand alone as far as the plot goes - just like HoS is, in fact.

GOG galaxy

Galaxy is NO game (if it were, it would be a lot more fun to install and use it) - the question had been about the version of the game, as in 1.10, 1.21, 1.somewhatelse.

But - what I told you works for every possible version (after HoS)...