Mihawk is above Yonko level confirmed in chapter 1058 but why Shanks?
Cuz I believe Oda took inspiration from Dragon Ball and created a Kami Character who split in Two (Piccolo)
I believe Shanks is Host of Good Kami of OPVerse, and the Final Villain is Evil Kami
This is why Oda always exaggerates in hyping just the Presence of Shanks and this is why Gorosei Respect him
Combat Wise he is Inferior to Mihawk, but Ability Wise he is as Strong as an Ancient Weapon
That's because just like Kami in Dragon Ball, he mainly tried to seal Piccolo but Combat Wise he was nerfed
To sum it up, in a 1v1 Fight Shanks is Inferior to Mihawk and as Strong as Roger, but against Armies and EOS Blackbeard or Final Villain, Shanks is Most Powerful
This actually was hinted in Thriller Bark, think of Mihawk as Zombie Ryuma and Shanks as Brook and EOS Blackbeard as Oars
Brook can never beat Zombie Ryuma in Combat, but Ryuma was weaker than Oars whom Brook almost killed using Ton of Salt, if not for Moria's Intervention
So Ryuma > Brook (Mihawk > Shanks) but the later have so much Salt (Refers to CoC) which makes him more Dangerous to Oars (EOS BB) and entire Thriller Bark (Refers to entire World)