I wouldn't trade Tkachuk for Marner, though it's very hard to argue at this stage of his career that he is "better". They Generate offense at similar levels, though Marner does it with an elite cast of offensive talent, and Brady is basically on an island, with virtually no help. For context, the worst C Marner ever had, Tyler Bozak in 2017, would have easily been the most offensively productive C Brady has ever played with at even strength.
The impact Brady has on his team's offensive production is also far more pronounced, because offense basically dries up entirely when he isn't out there. That said, Brady can't really compete with Marner's point totals, though I'm firmly in the goals mean more than assists camp.
Brady is one of the best in the league at generating not only shot volume, but also high danger chances. I don't think it's unfair to look at that, and expect him to score at a higher clip than the 25 goal pace he's been at for the first two years of his career. If he can give 30-35 goals and around 70 points, I don't think it's unfair to suggest that his skillset makes him more valuable than Marner is as a 20 goal, 90ish point guy.
Also worth considering that Brady is eligible to sign a new contract, and with him only being a 25/50 guy to this point, that contract is almost certainly coming in at significantly lower cost than Marner's.
I think Marner is the better player, but I don't think the gap is as significant as it seems based on point totals, and I think Brady will eventually be the better player. I don't think either team makes that trade.