David,
You’re well on your way and you’re obviously putting in the good work! 10 minutes a day is perfect. The big thing is that in your video you get it right a few times and that is proof that you can execute it. It’s now just a matter of making it consistent. In those ten minutes a day break up your practice (ten minutes on one thing is a lot!).
Start with just down strokes for 30 seconds or so
Then add the bum-ditty rhythm with the second down stroke and thumb for 30 seconds.
Rinse and repeat.
This will keep resetting you to that basic down stroke and feeling the beat which is the most important thing.
The other thing is take breaks every couple minutes. Take your hand off the instrument and relax. We all tend to carry a lot of tension in our hands when we’re learning a new technique and it’s important to take the hands off the instrument and just shake them out from time to time.
Cheers!
Christopher.