What part of his game is starting to come around that is reassuring for him to be the 2C? He's not doing anything offensively and is horrible defensively. If expectations are now for him to be a 3rd line checking player then I can agree that him getting involved physically more often now is a good sign, but that is not the player we wanted when we went after him.
Also yes you can't blame him for every single minus as per the Matheson turnover example you brought up, but how about dissecting every single minus he's received this season? I mean everyone else on that unit also received a minus so it's not like it was only Dach that got his stats hurt by a stupid Matheson play. Yet he's still the worst plus/minus in the league without hard minutes or matchups. Can't blame Matheson and others for that.
He's carrying the puck into traffic with more confidence, he's making more of the high end passes we've seen in the past, he's getting to the net with authority more often and starting shit in and around the net.
Those are the things that can make him a 2C, when that confidence comes in what his identity is and he drives the net more, him shooting more will open up even further.
He's had some atrocious moments defensively, especially at the beginning, but lately he is getting minuses even when he is playing the right way and defending the right way. It's just a way of saying, his plus/minus is bad but it looks a lot worse when sometimes it's other people doing batshit crazy things and you happen to be on the ice with that happening. Not every minus is created equal. Lately his game scores have been consistently pretty strong, so his advanced analytics are creeping up too, we know when he's on his game, the possession game is strong.
His game is slowly upticking, it might be a slow burn rather than everything clicking.