I would give my thoughts giving quick summary, let me know what you guys think
King vs Queen -> King low/mid diffs. Featwise its low diff because Queen can't do any damage to King, he can't dodge King's top speed, magna would burn him, explosions would kill him etc. But portrayal wise Queen the commander low/mid diffs Kaido's official right hand man
King vs Katakuri -> King mid/high diffs. Again Katakuri won't be doing any damage due to him lacking in AP and KIng has means to break FS with magma huge dragon, with tempuradon, with speed and with explosion. King would take his time one hour two hours maybe three hours but with his monsterous stamina he will win and its not a big deal.
King vs Marco --> Overall High diff. Marco needed devine thistle to block casual King fire punch. Marco was damaged by King on panel, King has superior stamina, phyiscally superior, both can regen and King fucked Marco up after their fight ended. Fresh Marco vs Fresh King is high diff for King
King vs Post Udon Luffy --> Once again high diff for King. Luffy can't hurt King effectively, Red Roc won't do much to King, he will regen through it. Magma will burn Luffy's arm, King can fight in range, in CqC, physically far superior bedies boundman.