Here's how I'd do the math based on 1760 yards in 1 mile:
1760 yards = 70.5 lengths in a 25 yard pool;
To complete 70.5 lengths in 1800 seconds (30 minutes) you need to swim each length in 25.5 seconds;
On each of those lengths you will probably use 3.5 seconds for pushing off and turning (1/2 of turn time is what I would factor into each length);
25.5 - 3.5 = 22.0 seconds of actual swim time per length to break 30 minutes.
To do that holding 17 strokes per length you need to swim at just under a 1:30 TT setting. At 18 strokes the tempo would be 1.22; at 19 the tempo is 1:15. If you want to hit 30 minutes at a 1.35 tempo you pretty much have to do it at 16 SPL. Of course, your push-off and turn time my actually consume more than 3.5 seconds. That means an even faster tempo is required to achieve your goal pace for each of these stroke counts.
If your push off/turn time is 4.5 seconds, that means your actual swim portion of each length goes down to 21.0 seconds. The new tempo goals per SPL:
16 SPL / 1:31
17 SPL / 1:23
18 SPL / 1:16
19 SPL / 1:10
The short answer, as AndyinNorway pointed out, is "if you want to swim faster swim faster." The trick is to manage it in a progressive way. Do you currently have a combination of SPL and TT that allows you to swim at 25.5 seconds per length? Let's say that right now you can do that at 18 SPL and 1.22 TT (using our 3.5 second push-off/turn number.) How long can you sustain that combination? For 50 yards? 200? 500? This will at least give you some insight to where you are now versus your goal. Do you have a combination that allows you to swim faster than your mile goal pace? You will need to do some of our training at that combination. Over time I would like to see two things happen: 1) You are able to sustain your desired combination for longer distances 2) Your combination becomes more efficient, so you can manage 17 SPL at 1:22 TT (one stroke less at the same tempo), or 18 SPL at 1:16 TT (the same stroke count at a higher tempo.)
Stroke mechanics obviously play a role in achieving your goal. To abandon low intensity drill and technique work completely would be a mistake. You can learn to eliminate a little more drag, or develop a more effective catch, or iron out small errors that occur when you breathe.
One other point: I'm a bit concerned that you "peter out" at a 1:30 tempo, which is pretty light. How do you manage your breathing? One breath every two strokes? Every 3rd? Less frequently than that? Here's why:
At a 1.3 tempo breathing every two strokes you get to breathe 23 times per minute (one breath every 2.6 seconds.) At a 1.3 tempo breathing every three strokes you get to breathe about 15 times per minute (one breath every 3.9 seconds.) That's not a lot of air. CPR used to call for 12 breaths per minute just to make sure the guy on the floor doesn't turn into a corpse. 15 breaths/minute is not enough air to sustain aerobic activity. Unless you count eating breakfast as an aerobic activity (I give myself 10 minutes of daily aerobic training credit for eating breakfast--haha!)