You have some conflicting parameters.
Like asking for 40 MPG and 5000 lb towing capacity. Nice,but not likely.
The human eye pupil is about 5mm in lower light,7mm in dark. That is the critical light "Bottleneck"
The scope will have an "Exit Pupil" You may be able to see a small dot of light if you hold the scope out at arms length and look at the ocular.
We are stuck with some math here.
You divide the Objective lense size by the magnification and you get the exit pupil size.
Using binoculars as an example,a 7x35 is regarded as a fine field magnification,as is an 8x40. Note the math will yield a 5mm exit pupil.
Now,for night/marine use,the lense size goes up,to 7x50 (49) or 8x56.
Note the 7mm exit pupil. The idea is we gather the light available on the area of the large objective lense and bend it down to a pupil the size of the human eye pupil.
There are other factors that can waste more or less light,like glass and lense coatings.Those factors aside, 7mm exit pupil,times the magnification,equals the optimum low light objective lense size.
You have a parameter of 12 X minimum. You have a need for max light transmission. For your optic to have a 5mm exit pupil,you would need a 60 mm objective at 12 X . For it to have a 7mm exit pupil,you would need an 84 mm objective.
In my limited experience,the best night performance scope I have is my Leupold fixed 6X by 42 mm. That's,IMO,plenty of magnification to shoot a prairie dog at 250 yds.I have used it in low light conditions. I was impressed. Its certainly better than naked eye. Amazing.But it has limits.And the reticle is invisible in darkness. But you are talking about legal hunting hours ,not night hunting. An illumined reticle can be useful in low light,,but it won't help counting antler points.
I don't like using my rifle scope for my spotting scope.
You might look at a 84 mm quality spotting scope ($ 1500 +) with a 12X eyepiece.
I'm not trying to be a smart alec,thats just the answer to your question
Optical quality costs money. I don't own a Leupold VX-2. I don't know what the optical quality is. Its pretty good on the 6x42. I think that's an FX-3 designation,but I forget. Leupold aside, I have a 1930's vintage Bausch and Lomb 60mm spotting scope. The "NRA" model. It has a 15X eyepiece.
On any typical range day in good light,using an orange and white target,I can see bullet holes at 200 yds well enough to sight in without going downrange to check my target.
There is more to resolving detail than magnification