Fighter/Rogue BBG fiddling

[Ketiara]

Im making a fighterrogue nemesis to my group of 4 lvl 17 players. (fiendwarlock, Eldrich knight, hunterRanger, Assassinrogue)

Im going 1thief/4champFighter/13thief, high elf, enforcerbackground.

Im kinda stealing him from a book, and then tweeking him to my game with 5e rules, and for this I need some help.

Im going thief 14 to get the ability to manipulate magic items out of my class, this guy has gathered many magic items to gain magical power in addition to fighting power. He has been successful in robbing the magicguild.
now he has Staff of power, cape of mountebank, belt of frost giant, headband of intellect and some scrolls/wands that I havent decided upon, but one of my questions is concerning the staff of power.
It gives me access to some spells, but as a thief I don't inherently have a casting ability, and thus no spell DC or Attack, I know im the DM so I can do as I please, but would it be viable for a thief to use the staff of powers spells?

And if yes, would his DC be 8+proff(6)+Int (+4)=18?

Lastly a level 17 npc (made with PC rules) with these items, how would you rate his CR?

And the thiefs ability to use a bonus action to use an object, how does that translate to magic items, scrolls, wands, staffs?

[Ketiara]

Okay perhaps my questions wasn't clear, so ill be more specific.

Is it RAW for a thief's "Use magic device" to use the staff of powers spells, and what would the DC on spells be?
And the thief's ability to use a bonus action "use an object", does that include scrolls, wands, rods, staffs?

Hope my English is sufficient, it isn't my native language.

[Osrogue]

Okay perhaps my questions wasn't clear, so ill be more specific.

Is it RAW for a thief's "Use magic device" to use the staff of powers spells, and what would the DC on spells be?
And the thief's ability to use a bonus action "use an object", does that include scrolls, wands, rods, staffs?

Hope my English is sufficient, it isn't my native language.

Use magic device let's the rogue use the items as a member of that class would. So the DC would be 8+6(proficiency)+4(int)

Magic items cannot be used with a bonus action f they normally require an action.

[Ketiara]

Use magic device let's the rogue use the items as a member of that class would. So the DC would be 8+6(proficiency)+4(int)

Magic items cannot be used with a bonus action f they normally require an action.

cool thanks for the answer