I think you may be over-thinking this a bit. As Piscean mentioned earlier, these aren't conscious decision we're making as we speak. Plus, different speakers, particularly those with different variants, or even regional variants, may articulate slightly differently.
For me, I pronounce the 'th' sound purely as dental - you can see my tongue stick out between my teeth. I don't know if you could even measure the fractional millisecond of time you might potentially save transitioning from alveolar /n/ to alveolar 'th' versus dental 'th'. I mean, you're talking maybe a millimeter or two difference between /n/ and 'th'. Alveolar isn't where the teeth enter the gum, alveolar is the ridge or bump made the roots of your teeth, where the gum rounds up into the top of the mouth (or palate). Take a look at
this map - notice alveolar sounds are made around positions 4 and 5.
My /n/ is so far back on the alveolar ridge as to be almost palatial. Still, that jump from high alveolar to dental can't be measured or noticed, at least not with extensive sound analysis!
But still, I think it's quibbling to focus on these minutiae. Better to just practice the transition hundreds of times. Eventually it'll become so natural and fluid you won't be concentrating on where you tongue is.
Start slow, and make a distinct, separate /n/ sound. Then slowly make your 'th' sound. Slowly alternate between the two for a couple dozen times. Gradually start picking up speed as you alternate between them until you have a seamless transition. Then practice that blend a hundred times.
At that point, it'll be so automatic the two sounds will be almost one. While you're practicing, focus not on the tongue position or time, but just on the final sound.