Mariners Acquire Spencer Steer and Jake Fraley/Will Benson for Bryce Miller/Bryan Woo and Jonatan Clase
I think the Mariners would entertain trading Miller for Steer, but the Reds would need to include more for one of the Mariners young starters. I don't know if the Reds would be willing to move Steer and another outfielder like a Jake Fraley or Will Benson, but it might take someone like that to sweeten the deal for the Mariners.
This gives the Mariners a high-end bat in Steer to rotate between the corner infield and outfield spots and DH while slotting in the middle of the order next to Julio, Cal, and Geno. They also get a solid left-handed outfielder to compete with Dominic Canzone for that 4th outfield spot, presuming they add a bigger bat to start in the corner opposite of Kelenic.
This gives the Reds a starter with legitimate #2 upside. Miller is probablty slightly more valuable due to the injury concerns around Bryan Woo. But either one would most likely slot in at the #2 or #3 spot behind Hunter Green and Andrew Abbott.