My take on his defensive potential is that he doesn't have great upside there but he has plenty of room to not be a liability. With his upside on the offensive end, being a solid defender is all you really need to have a high level player. There's enough size, athleticism and awareness to be a guy that you don't have to hide on the defensive end but you'll probably want to pair him with a better wing defender for the quality offensive wings. For me, Frank/Doncic/KP gives you a top three with two passers (Doncic/Frank), two good scorers (KP/Doncic), and two guys with elite potential on defense (Frank/KP) without ever mentioning that all three should be good floor spacers. You may have to tinker with the pieces between them but you can slot in a lot of skillsets with those dudes thanks to their size, length and complimentary talents.
To be fair, I see a lot of this in Bagley too as far as the fit goes. My preference is Doncic but Bagley has a strong case himself when we talk fit.