Bar mitts (aka pogies) with light full finger gloves are, as the Gen-Zers say, amazeballs. You get good brake and shift actuation with the lighter gloves. Or manipulate head unit buttons or touchscreens easily. And the pogies can be great. In mountain biking in cold weather with full heavy gloves, I'd be fine ascending, but my hands would absolutely freeze descending. Like after the ride I was unable to work my helmet buckle or enter the code to my garage door opener. But with pogies, wind from descending is zero issue.

Edit: But I'm talking maybe more single-digit to teens F, not 30F-45F. For that you might be better off with just medium-weight full finger gloves. Or sometimes if there's a descent late in a mountain day, I'll keep a pair of dry glove liners in my jersey pocket, and throw those on under the medium gloves for the descent.