Joshua's been very good this year, but there's absolutely a chance it's a flash in the pan. As you've said, it's hard to gauge how much of this year's success is representative of who he actually is as a player. Yeah, he's physical, has solid speed, and has put up solid production. He's been very good in the playoffs thus far, but we've seen guys get paid for one off playoff performances too.
I think it makes sense for him to chase the bag via UFA, he could probably get more elsewhere and given his career earnings thus far he'd probably be smart to do so. I'm not as concerned about slotting another player into that spot as some are, his physicality can be replaced by someone else, or collectively.
Zadorov is different as you've said, and we're more likely to get what we've gotten from him this season going forward. If he's willing to take five years and 5M or less that's pretty solid. It'd be interesting to see him on the powerplay, sure, but I'm not sure we'd see it, NHL teams tend to go with more forward heavy powerplay units now. It's often four forwards and a D.