The pains are normal, although the Achilles is troubling. Watch it carefully.
I've suggested the "Elbows back" drill, but one of your friends suggested you focus on the arm downswing. Try that too. Sometimes, thinking about a different part of the arm motion creates the aha moment when it feels right.
Ideally, your next step would be to try your new form at mile or 5k goal pace. I would suggest the Billat 30-30 workout, especially if you can run outdoors or at an indoor track. After warming up, you would practice your new form at mile or 5k race pace for 30 seconds, then jog slowly with your normal form for 30 seconds, then repeat the process. Start with only 4 reps and then stop and walk to see how you feel. If you don't have Achilles pain, do another set of 4. The slow jog should be really slow. You can even walk if you find yourself having trouble holding the new form for 30 seconds.
I teach this workout to adult runners to help them improve their efficiency at faster speeds, since many recreational runners seldom run faster than marathon pace, but they tend to get hurt if they jump directly into interval workouts.
You can do this on a treadmill, but you'll need to add extra time to speed up and slow down the treadmill, so maybe 40 seconds. The recovery can be as long as you need to run the next rep in good form... say one minute to 90 seconds.
Gradually, you increase the volume in this workout. There is no need to increase speed faster than mile race pace. For example, when coming back from an MCL injury, I first ran 1 set of 8x30(30) on my first day back on the track. The next week I ran 2 sets of 8x30(30). The next week, 17x30/30 and then 26x30/30.
At this point, you're doing Billats AS A DRILL, but eventually, you'll be able to do it as an interval workout. As a workout, you would run 30 seconds at mile race pace or slightly faster, then jog 30 seconds at half that speed, then repeat until you can no longer hold mile race pace for 30 seconds. Your goal is 20 to 24 reps.
If you do Billats on a track, you can measure weekly progress by tracking how far you run in a specific number of reps. You could also do it with a Garmin on the road.