Maximising drive space

I’m rebuilding my NAS from an existing machine with 6 x 4tb with 2x 18tb. I wanted to set things up as a mirror (i think, not sure on the best layout for current drives) but I think I’ll lose a lot of space if i pool everything together with the mixed drives. Should I do 2 separate pools instead?

Long term I’ll replace the 4tb with 18tb drives but that’s a way off