Usopp is neither senior in power nor authority, he's a weak fodder and he'll always be a weak fodder, hence the weak trio, a canonical group that he's a part of, Luffy's most senior officers are Zoro, Sanji and Jinbe.
Those 3 + Yamato would be quite the shield. Just ask yourselves, if Alabasta or Amazon Lily would have been attacked again - would Luffy trust Usopp or one of his top 4 to handle it?
If not for Green Bull Yamato would already have joined, it speaks volumes of how high Luffy rates her that he knew she could handle almost any enemy on her own.
To be honest it goes
1 Zoro - chocked when he lost to Enel. Trusts him with any enemy, even Kaido & Big Mom, until he needed rest.
2 Yamato - trusted to take care of Wano on his behalf against likely strong foes.
3 Jinbei - trusted him all the way to the roof top. Only hesitant vs Big Mom.
4 Sanji - berated him for being slow on way to roof top and at the moment doesnt trust him as much as the top 3.