I am glad the conversation got around to contact, feeling and dwell time. In my opinion, that is really where your serves get more spin. The contact you want is a lot like the contact you want for doing a touch loop against underspin. When you do that, what you do is try to contact the edge of the ball. A lot of people at the club I play at call this "brush contact". It is sort of right but not detailed enough. What you are trying to do is catch just the edge of the ball but you are trying to let the ball sink in to the rubber and you are making solid contact with the edge of the ball rather than brushing past it. As you catch the edge of the ball and let the ball sink in to the rubber, you feel for when the rubber grabs the ball. Then, when the rubber has grabbed the ball you accelerate the stroke, or snap; and here is where you would add extra motion from your wrist: not before. When you do that you get a ton of spin on the ball on your loop.
When you are serving you want to do something very similar. There are a few drills I do for looping underspin and when I have done them and then I practice my serves I notice that I just naturally am getting way more spin than normal. At a certain point I think that just becomes natural and you don't need the drills to find that contact as you are warming up. So practicing serving could help your ability to loop, and practicing looping underspin could help your ability to get more spin on the serves.
The main differences between looping underspin and serving are obvious. When you serve, the ball does not have spin of its own, you are tossing the ball to yourself and the stroke is a different motion. But how you contact the ball and how you generate spin are the same. When you contact the ball, you are looking to make contact with the edge of the ball. You want to feel the edge of the ball sink in to the rubber and you want to feel the rubber grab the ball. Then you want to accelerate your stroke. It does not matter how you generate that acceleration, the speed is actually not as important as the acceleration: that the racket is going from a slower speed to a faster speed while the ball is on the rubber. A wrist snap works really well, but in some of those shovel serve variations, there is very little movement from the wrist and there is still a ton of spin. However, the wrist motion is probably the easiest way to get that acceleration and that snap.
If you get that contact, it feels like you can do almost anything with the racket to get a lot of spin. However, if you have the movement in place, get the contact and time the snap of the wrist so that the blade accelerates after you contact the ball and the rubber has grabed the ball, you are going to get a lot of spin. Then when you do your no-spin serves they will be very effective as well.