Can I presume Solasta is to BG3 what Icewind Dale was to BG1+2?
No, you can't.
BG3 in it's current state is a beautifull modern game for casual/modern gamers that don't want to think too much or for players that want to play with friends. Deeper companion and story pathes but poor tactical/DnD combats, theme park map, classes all played the same,...
Solasta has deeper and consistent mechanics that works as a whole. It makes you live a journey accross it's world rather than throwing you "fun gameplay" on your face everytime for the sake of it. Despite the size of the project and the custom world, Solasta is far more like BG1/2 than BG3. The only things it lacks is freedom and a deeper story.
According to me BG3 has close to nothing from BG1/2 exept a bit of lore and names. These games aren't the same at all even if they uses DnD.