2.1 Thunder Burt on 21 internal width rim?

I have a front Syncros Capital 1.0 35 that's just sitting around. I'd like to use something wider than my current Terra speed's in 45mm.

Question for the community - how well would a 2.1 MTB tire work on an internal 21mm rim? I've read that MTB tires actually work better on narrow rims when compared to gravel tries due to a more flexible sidewall. Seeking recommendations, thanks.