All of the below is no professional advice, I’m just a programmer who looked into this a bit.

The easiest thing in my opinion would be to incorporate in Croatia and then use Paddle https://www.paddle.com/, Gumroad https://gumroad.com/ or FastSpring https://fastspring.com/ for payments. They take care of all your VAT/Sales taxes.

If you want to go with a simple company outside of Croatia, maybe go with a UK Ltd, easy and cheap to set up and run. I used Sleek https://sleek.com/uk/ for that.

But if you live in Croatia, I would go with a local equivalent of a Ltd + Paddle/Gumroad/FastSpring.