Finishing attic with static vent
The Code says Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.)
However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:

a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or

not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “  (See ICC R806.1.2.)


Currently your vents satisfy the “cross-ventilation requirement, but when you cover the roof rafters you will no longer have cross-ventilation.
Also, the Code says you shall maintain a 1” clearance between attic insulation and framing.
Where you live, R-19 batt insulation is grossly inadequate. (In fact, the 6” batt insulation will completely fill up the 2x6 joist space and you cannot vent it.) I’d use R-30 insulation (10” thick), but you’ll need to sister 2x12’s to the existing 2x6’s so you’ll have about 1 1/4” air space above the R-30 insulation. You’ll also need to provide soffit vents and a ridge vent (or provide a means for the air to cross the ridge.)
So, you can see there is clearly a benefit to having ridge vents, soffit vents, etc. , but in a proportion that allows COMPLETE CROSS-VENTILATION above your new insulation.