Your observations match what I've seen, completely. I've seen Marte live a few times now - at a handful of spring training games this year and last, and the past two games here in Chicago. He definitely looks more comfortable in the field now than when I saw him muff a few grounders in ST. I also notice an interesting dynamic between him and Cano. There's been a handful of plays (infield popups, DP turns, etc) where Cano plays around with him like he does Cruz. i.e. if Marte calls him off for a popup, Cano will just stand right next to him staring at him, and when the play is over start giving him smack talk (this is all playful). All of that to say, he never did that stuff with Miller or Taylor... Lemme just check the B-Ref page here... Yup, Ketel is from the DR :) So I think there's a cool dynamic happening on the dirt between the 21 year old kid and one of the all time great DR players.
And as for his strike zone control, it's been fun to watch compared to a lot of the other young guys over the past few years. Ketel, as you said, knows how to hit and control the zone front to back. With most of our other hitters, except maybe Cano / Cruz / Seager / Smith, everyone else is guessing. Sure they might hit 20 bombs, but that's just guessing right. With the other dudes, they are HITTING. Anyways, it's refreshing. I hope he builds on it.