Yes, this is for sure the best option. It's very easy to go from Gothenburg to Varberg by train, takes like 1 hour.
Quick and cheap. You can usually fly with Wizzair for about 100-150 zl.
The other options you mentioned are very inconvenient.
The downside is that these flights are just on Mondays and Fridays, and that they leave already at 6 in the morning from Warsaw. There are also planes between Gdańsk and Gothenburg on Tuesdays and Saturdays. These tickets are almost for free if you book quite a while in advance. Planes also go on Sundays, Mondays, Wednesdays and Fridays between Malmö and Warsaw. All Wizzair. Malmö is something like 2.5 hours by train from Varberg.
As for trains, I checked sj.se, and it's only supposed to take 35 minutes from Göteborg Central station to Varberg, and the price is less than 100 crowns (probably lower if you buy them early).