I will say imo, the Championship is a higher level than the Swiss League. What proof? The vast quality of players (and internationals). Where does my conclusion come from? The greater amount of players that move onto D1 leagues (England, France, Italy, Portugal, Germany) that are deemed higher levels than the Swiss League.
So for me, to see Millar performing at this level, and at a promotion fighting club is better than being at Basel. If he continues to perform, he'll be in the premier league, and if not a league better than the Swiss league (respectively) and that's all he could ask - trajectory.