A thing about making mittens that is maybe not obvious is that much of the warmth of mittens comes from the air trapped inside the mitten. Therefore, close-fitting mittens are not as warm.
This is especially noticeable in the thumb (where I accidentally took too much seam allowance. Don't do this.)
@Betty Do you know alien hoof mittens? Those are great (I have them but no pic right now), they are basically 3 finger gloves - thumb is normal, then index and middle finger have one large finger and ring finger and pinky go into another. Warmth of mittens but slightly more dexterity.
@Antimony I think of those as vulcan salute mittens.
@Betty too close fitting also means you can’t shove em over a pair of gloves, compounding the difficulties
@Satsuma I'm very into layering mittens for different amounts of cold.